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swig/Doc/Manual/Cpp0x.html
William S Fulton 6a43934ece Update variadic templates 
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>SWIG and C++0x</title>
<link rel="stylesheet" type="text/css" href="style.css">
</head>
<body bgcolor="#ffffff">
<H1><a name="Cpp0x"></a>7 SWIG and C++0x</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Cpp0x_Introduction">Introduction</a>
<li><a href="#Cpp0x_Core_language_changes">Core language changes</a>
<ul>
<li><a href="#Cpp0x_Rvalue_reference_and_move_semantics">Rvalue reference and move semantics</a>
<li><a href="#Cpp0x_Generalized_constant_expressions">Generalized constant expressions</a>
<li><a href="#Cpp0x_Extern_template">Extern template</a>
<li><a href="#Cpp0x_Initializer_lists">Initializer lists</a>
<li><a href="#Cpp0x_Uniform_initialization">Uniform initialization</a>
<li><a href="#Cpp0x_Type_inference">Type inference</a>
<li><a href="#Cpp0x_Range_based_for_loop">Range-based for-loop</a>
<li><a href="#Cpp0x_Lambda_functions_and_expressions">Lambda functions and expressions</a>
<li><a href="#Cpp0x_Alternate_function_syntax">Alternate function syntax</a>
<li><a href="#Cpp0x_Object_construction_improvement">Object construction improvement</a>
<li><a href="#Cpp0x_Null_pointer_constant">Null pointer constant</a>
<li><a href="#Cpp0x_Strongly_typed_enumerations">Strongly typed enumerations</a>
<li><a href="#Cpp0x_Double_angle_brackets">Double angle brackets</a>
<li><a href="#Cpp0x_Explicit_conversion_operators">Explicit conversion operators</a>
<li><a href="#Cpp0x_Alias_templates">Alias templates</a>
<li><a href="#Cpp0x_Unrestricted_unions">Unrestricted unions</a>
<li><a href="#Cpp0x_Variadic_templates">Variadic templates</a>
<li><a href="#Cpp0x_New_string_literals">New string literals</a>
<li><a href="#Cpp0x_User_defined_literals">User-defined literals</a>
<li><a href="#Cpp0x_Thread_local_storage">Thread-local storage</a>
<li><a href="#Cpp0x_Defaulting/deleting_of_standard_functions_on_C++_objects">Defaulting/deleting of standard functions on C++ objects</a>
<li><a href="#Cpp0x_Type_long_long_int">Type long long int</a>
<li><a href="#Cpp0x_Static_assertions">Static assertions</a>
<li><a href="#Cpp0x_Allow_sizeof_to_work_on_members_of_classes_without_an_explicit_object">Allow sizeof to work on members of classes without an explicit object</a>
</ul>
<li><a href="#Cpp0x_Standard_library_changes">Standard library changes</a>
<ul>
<li><a href="#Cpp0x_Threading_facilities">Threading facilities</a>
<li><a href="#Cpp0x_Tuple_types">Tuple types and hash tables</a>
<li><a href="#Cpp0x_Regular_expressions">Regular expressions</a>
<li><a href="#Cpp0x_General_purpose_smart_pointers">General-purpose smart pointers</a>
<li><a href="#Cpp0x_Extensible_random_number_facility">Extensible random number facility</a>
<li><a href="#Cpp0x_Wrapper_reference">Wrapper reference</a>
<li><a href="#Cpp0x_Polymorphous_wrappers_for_function_objects">Polymorphous wrappers for function objects</a>
<li><a href="#Cpp0x_Type_traits_for_metaprogramming">Type traits for metaprogramming</a>
<li><a href="#Cpp0x_Uniform_method_for_computing_return_type_of_function_objects">Uniform method for computing return type of function objects</a>
</ul>
</ul>
</div>
<!-- INDEX -->
<H2><a name="Cpp0x_Introduction"></a>7.1 Introduction</H2>
<p>This chapter gives you a brief overview about the SWIG
implementation of the C++0x standard. This part of SWIG is still a work in
progress. Initial C++0x support for SWIG was written during the
Google Summer of Code 2009 period.</p>
<p>SWIG supports all the new C++ syntax changes with some minor limitations
(decltype expressions, variadic templates number). Wrappers for the
new STL types (unordered_ containers, result_of, tuples) are not supported
yet.</p>
<H2><a name="Cpp0x_Core_language_changes"></a>7.2 Core language changes</H2>
<H3><a name="Cpp0x_Rvalue_reference_and_move_semantics"></a>7.2.1 Rvalue reference and move semantics</H3>
<p>SWIG correctly parses the new operator &amp;&amp; the same as the reference operator &amp;.</p>
<p>The wrapper for the following code is correctly produced:</p>
<div class="code"><pre>
class MyClass {
MyClass(MyClass&amp;&amp; p) : ptr(p.ptr) {p.ptr = 0;}
MyClass&amp; operator=(MyClass&amp;&amp; p) {
std::swap(ptr, p.ptr);
return *this;
}
};
</pre></div>
<H3><a name="Cpp0x_Generalized_constant_expressions"></a>7.2.2 Generalized constant expressions</H3>
<p>SWIG correctly parses the keyword <tt>constexpr</tt>, but ignores its functionality. Constant functions cannot be used as constants.</p>
<div class="code"><pre>
constexpr int myConstFunc() { return 10; }
const int a = myConstFunc(); // results in error
</pre></div>
<p>Users needs to use values or predefined constants when defining the new constant value:</p>
<div class="code"><pre>
#define MY_CONST 10
constexpr int myConstFunc() { return MY_CONST; }
const int a = MY_CONST; // ok
</pre></div>
<H3><a name="Cpp0x_Extern_template"></a>7.2.3 Extern template</H3>
<p>SWIG correctly parses the keywords <tt>extern template</tt>. However, the explicit template instantiation is not used by SWIG, a <tt>%template</tt> is still required.</p>
<div class="code"><pre>
extern template class std::vector&lt;MyClass&gt;; // explicit instantiation
...
class MyClass {
public:
int a;
int b;
};
</pre></div>
<H3><a name="Cpp0x_Initializer_lists"></a>7.2.4 Initializer lists</H3>
<p>Constructors using the std::initializer_list class are removed
from the wrapped class, because the only way to access such a
constructor is at compile time using the "= {}" assignment.</p>
<p>Users should add another constructor with specific arguments
filling the class members manually.</p>
<p>For now, if a user wants to fill the class components like this:</p>
<div class="code"><pre>
class A {
public:
A( std::initializer_list&lt;int&gt; );
};
A a1 = {1,2,3,4};
</pre></div>
<p>You should add another constructor using the std::vector for example:</p>
<div class="code"><pre>
class A {
public:
A( std::initializer_list&lt;int&gt; );
A( std::vector&lt;int&gt; );
};
A a1 = {1,2,3,4};
</pre></div>
<p>And call it from your target language, for example, in Python:</p>
<div class="targetlang"><pre>
&gt;&gt;&gt; a2 = A( [1,2,3,4] )
</pre></div>
<H3><a name="Cpp0x_Uniform_initialization"></a>7.2.5 Uniform initialization</H3>
<p>The curly brackets {} for member initialization are fully
supported by SWIG:</p>
<div class="code"><pre>
struct BasicStruct {
int x;
double y;
};
struct AltStruct {
AltStruct(int x, double y) : x_{x}, y_{y} {}
int x_;
double y_;
};
BasicStruct var1{5, 3.2}; // only fills the struct components
AltStruct var2{2, 4.3}; // calls the constructor
</pre></div>
<p>Uniform initialization does not affect usage from the target language, for example in Python:</p>
<div class="targetlang"><pre>
&gt;&gt;&gt; a = AltStruct(10, 142.15)
&gt;&gt;&gt; a.x_
10
&gt;&gt;&gt; a.y_
142.15
</pre></div>
<H3><a name="Cpp0x_Type_inference"></a>7.2.6 Type inference</H3>
<p>SWIG supports <tt>decltype()</tt> with some limitations. Single
variables are allowed, however, expressions are not supported yet. For
example, the following code will work:</p>
<div class="code"><pre>
int i;
decltype(i) j;
</pre></div>
<p>However, using an expression inside the decltype results in syntax error:</p>
<div class="code"><pre>
int i; int j;
decltype(i+j) k; // syntax error
</pre></div>
<H3><a name="Cpp0x_Range_based_for_loop"></a>7.2.7 Range-based for-loop</H3>
<p>This feature is part of the implementation block only. SWIG
ignores it.</p>
<H3><a name="Cpp0x_Lambda_functions_and_expressions"></a>7.2.8 Lambda functions and expressions</H3>
<p>SWIG correctly parses most of the Lambda functions syntax. For example:</p>
<div class="code"><pre>
auto val = [] { return something; };
auto sum = [](int x, int y) { return x+y; };
auto sum = [](int x, int y) -&gt; int { return x+y; };
</pre></div>
<p>The lambda functions are removed from the wrapper class for now, because of the lack of support
for closures (scope of the lambda functions) in the target languages.</p>
<p>
Lambda functions used to create variables can also be parsed, but due to limited support of <tt>auto</tt> when
the type is deduced from the expression, the variables are simply ignored.
</p>
<div class="code"><pre>
auto six = [](int x, int y) { return x+y; }(4, 2);
</pre></div>
<p>
Better support should be available in a later release.
</p>
<H3><a name="Cpp0x_Alternate_function_syntax"></a>7.2.9 Alternate function syntax</H3>
<p>SWIG fully supports the new definition of functions. For example:</p>
<div class="code"><pre>
struct SomeStruct {
int FuncName(int x, int y);
};
</pre></div>
<p>can now be written as in C++0x:</p>
<div class="code"><pre>
struct SomeStruct {
auto FuncName(int x, int y) -&gt; int;
};
auto SomeStruct::FuncName(int x, int y) -&gt; int {
return x + y;
}
</pre></div>
<p>The usage in the target languages remains the same, for example in Python:</p>
<div class="targetlang"><pre>
&gt;&gt;&gt; a = SomeStruct()
&gt;&gt;&gt; a.FuncName(10,5)
15
</pre></div>
<p>SWIG will also deal with type inference for the return type, as per the limitations described earlier. For example:</p>
<div class="code"><pre>
auto square(float a, float b) -&gt; decltype(a);
</pre></div>
<H3><a name="Cpp0x_Object_construction_improvement"></a>7.2.10 Object construction improvement</H3>
<p>
SWIG is able to handle constructor delegation, such as:
</p>
<div class="code"><pre>
class A {
public:
int a;
int b;
int c;
A() : A( 10 ) {}
A(int aa) : A(aa, 20) {}
A(int aa, int bb) : A(aa, bb, 30) {}
A(int aa, int bb, int cc) { a=aa; b=bb; c=cc; }
};
</pre></div>
<p>
Constructor inheritance is parsed correctly, but the additional constructors are not currently added to the derived proxy class in the target language. Example is shown below:
<!--
The extra constructors provided by the <tt>using</tt> syntax will add the appropriate constructors into the target language proxy derived classes.
In the example below a wrapper for the <tt>DerivedClass(int)</tt> is added to <tt>DerivedClass</tt>:
-->
</p>
<div class="code"><pre>
class BaseClass {
public:
BaseClass(int iValue);
};
class DerivedClass: public BaseClass {
public:
using BaseClass::BaseClass; // Adds DerivedClass(int) constructor
};
</pre></div>
<H3><a name="Cpp0x_Null_pointer_constant"></a>7.2.11 Null pointer constant</H3>
<p>The <tt>nullptr</tt> constant is largely unimportant in wrappers. In the few places it has an effect, it is treated like <tt>NULL</tt>.</p>
<H3><a name="Cpp0x_Strongly_typed_enumerations"></a>7.2.12 Strongly typed enumerations</H3>
<p>SWIG parses the new <tt>enum class</tt> syntax and forward declarator for the enums:</p>
<div class="code"><pre>
enum class MyEnum : unsigned int;
</pre></div>
<p>The strongly typed enumerations are treated the same as the ordinary and anonymous enums.
This is because SWIG doesn't support nested classes. This is usually not a problem, however,
there may be some name clashes. For example, the following code:</p>
<div class="code"><pre>
class Color {
enum class PrintingColors : unsigned int {
Cyan, Magenta, Yellow, Black
};
enum class BasicColors {
Red, Green, Blue
};
enum class AllColors {
// produces warnings because of duplicate names
Yellow, Orange, Red, Magenta, Blue, Cyan, Green, Pink, Black, White
};
};
</pre></div>
<p>A workaround is to write these as a series of separated classes containing anonymous enums:</p>
<div class="code"><pre>
class PrintingColors {
enum : unsigned int {
Cyan, Magenta, Yellow, Black
};
};
class BasicColors {
enum : unsigned int {
Red, Green, Blue
};
};
class AllColors {
enum : unsigned int {
Yellow, Orange, Red, Magenta, Blue, Cyan, Green, Pink, Black, White
};
};
</pre></div>
<H3><a name="Cpp0x_Double_angle_brackets"></a>7.2.13 Double angle brackets</H3>
<p>SWIG correctly parses the symbols &gt;&gt; as closing the
template block, if found inside it at the top level, or as the right
shift operator &gt;&gt; otherwise.</p>
<div class="code"><pre>
std::vector&lt;std::vector&lt;int&gt;&gt; myIntTable;
</pre></div>
<H3><a name="Cpp0x_Explicit_conversion_operators"></a>7.2.14 Explicit conversion operators</H3>
<p>SWIG correctly parses the keyword <tt>explicit</tt> both for operators and constructors.
For example:</p>
<div class="code"><pre>
class U {
public:
int u;
};
class V {
public:
int v;
};
class TestClass {
public:
//implicit converting constructor
TestClass( U const &amp;val ) { t=val.u; }
// explicit constructor
explicit TestClass( V const &amp;val ) { t=val.v; }
int t;
};
</pre></div>
<p>
The usage of explicit constructors and operators is somehow specific to C++ when assigning the value
of one object to another one of different type or translating one type to another. It requires both operator and function overloading features,
which are not supported by the majority of SWIG target languages. Also the constructors and operators are not particulary useful in any
SWIG target languages, because all use their own facilities (eg. classes Cloneable and Comparable in Java)
to achieve particular copy and compare behaviours.
</p>
<H3><a name="Cpp0x_Alias_templates"></a>7.2.15 Alias templates</H3>
<p>
The following is an example of an alias template:
<div class="code"><pre>
template&lt; typename T1, typename T2, int &gt;
class SomeType {
T1 a;
T2 b;
int c;
};
template&lt; typename T2 &gt;
using TypedefName = SomeType&lt;char*, T2, 5&gt;;
</pre></div>
<p>
These are partially supported as SWIG will parse these and identify them, however, they are ignored as they are not added to the type system. A warning such as the following is issued:
</p>
<div class="shell">
<pre>
example.i:13: Warning 342: The 'using' keyword in template aliasing is not fully supported yet.
</pre>
</div>
<p>
Similarly for non-template type aliasing:
</p>
<div class="code"><pre>
using PFD = void (*)(double); // New introduced syntax
</pre></div>
<p>
A warning will be issued:
</p>
<div class="shell">
<pre>
example.i:17: Warning 341: The 'using' keyword in type aliasing is not fully supported yet.
</pre>
</div>
<p>The equivalent old style typedefs can be used as a workaround:</p>
<div class="code"><pre>
typedef void (*PFD)(double); // The old style
</pre></div>
<H3><a name="Cpp0x_Unrestricted_unions"></a>7.2.16 Unrestricted unions</H3>
<p>SWIG fully supports any type inside a union even if it does not
define a trivial constructor. For example, the wrapper for the following
code correctly provides access to all members in the union:</p>
<div class="code"><pre>
struct point {
point() {}
point(int x, int y) : x_(x), y_(y) {}
int x_, y_;
};
#include <new> // For placement 'new' in the constructor below
union P {
int z;
double w;
point p; // Illegal in C++03; legal in C++11.
// Due to the point member, a constructor definition is required.
P() {
new(&amp;p) point();
}
} p1;
</pre></div>
<H3><a name="Cpp0x_Variadic_templates"></a>7.2.17 Variadic templates</H3>
<p>SWIG supports the variadic templates syntax (inside the &lt;&gt;
block, variadic class inheritance and variadic constructor and
initializers) with some limitations. The following code is correctly parsed:</p>
<div class="code"><pre>
template &lt;typename... BaseClasses&gt; class ClassName : public BaseClasses... {
public:
ClassName (BaseClasses&amp;&amp;... baseClasses) : BaseClasses(baseClasses)... {}
}
</pre></div>
<p>Support for the variadic sizeof() function was also introduced:</p>
<div class="code"><pre>
const int SIZE = sizeof...(ClassName&lt;int, int&gt;);
</pre></div>
<p>
For now however, the <tt>%template</tt> directive only accepts one parameter substitution
for the variable template parameters.
</p>
<div class="code"><pre>
%template(MyVariant1) ClassName&lt;&gt; // zero argument not supported
%template(MyVariant2) ClassName&lt;int&gt; // ok
%template(MyVariant3) ClassName&lt;int, int&gt; // too many arguments
</pre></div>
<H3><a name="Cpp0x_New_string_literals"></a>7.2.18 New string literals</H3>
<p>SWIG fully supports unicode string constants and raw string literals.</p>
<div class="code"><pre>
// New string literals
wstring aa = L"Wide string";
const char *bb = u8"UTF-8 string";
const char16_t *cc = u"UTF-16 string";
const char32_t *dd = U"UTF-32 string";
// Raw string literals
const char *xx = ")I'm an \"ascii\" \\ string.";
const char *ee = R"XXX()I'm an "ascii" \ string.)XXX"; // same as xx
wstring ff = LR"XXX(I'm a "raw wide" \ string.)XXX";
const char *gg = u8R"XXX(I'm a "raw UTF-8" \ string.)XXX";
const char16_t *hh = uR"XXX(I'm a "raw UTF-16" \ string.)XXX";
const char32_t *ii = UR"XXX(I'm a "raw UTF-32" \ string.)XXX";
</pre></div>
<p>Note: SWIG currently incorrectly parses the odd number of double quotes
inside the string due to SWIG's C++ preprocessor.</p>
<H3><a name="Cpp0x_User_defined_literals"></a>7.2.19 User-defined literals</H3>
<p>
SWIG parses the declaration of user-defined literals, that is, the <tt>operator "" _mysuffix()</tt> function syntax.
</p>
<p>
Some examples are the raw literal:
</p>
<div class="code"><pre>
OutputType operator "" _myRawLiteral(const char * value);
</pre></div>
<p>
numeric cooked literals:
</p>
<div class="code"><pre>
OutputType operator "" _mySuffixIntegral(unsigned long long);
OutputType operator "" _mySuffixFloat(long double);
</pre></div>
<p>
and cooked string literals:
</p>
<div class="code"><pre>
OutputType operator "" _mySuffix(const char * string_values, size_t num_chars);
OutputType operator "" _mySuffix(const wchar_t * string_values, size_t num_chars);
OutputType operator "" _mySuffix(const char16_t * string_values, size_t num_chars);
OutputType operator "" _mySuffix(const char32_t * string_values, size_t num_chars);
</pre></div>
<p>
Note that the %rename directive currently does not parse the double quotes, so these can't be easily accessed from
target languages.
</p>
<p>
Use of user-defined literals such as the following still give a syntax error:
</p>
<div class="code"><pre>
OutputType var1 = "1234"_suffix;
OutputType var2 = 1234_suffix;
OutputType var3 = 3.1416_suffix;
</pre></div>
<H3><a name="Cpp0x_Thread_local_storage"></a>7.2.20 Thread-local storage</H3>
<p>SWIG correctly parses the <tt>thread_local</tt> keyword. For example, a variable
reachable by the current thread can be defined as:</p>
<div class="code"><pre>
struct A {
thread_local int val;
};
</pre></div>
<p>The new C++0x threading libraries are ignored because each SWIG target language offers
its own threading facilities.</p>
<H3><a name="Cpp0x_Defaulting/deleting_of_standard_functions_on_C++_objects"></a>7.2.21 Defaulting/deleting of standard functions on C++ objects</H3>
<p>SWIG correctly parses the <tt>= delete</tt> and <tt>= default</tt>
keywords. For example:</p>
<div class="code"><pre>
struct NonCopyable {
NonCopyable&amp; operator=(const NonCopyable&amp;) = delete; /* Removes operator= */
NonCopyable(const NonCopyable&amp;) = delete; /* Removed copy constructor */
NonCopyable() = default; /* Explicitly allows the empty constructor */
void *operator new(std::size_t) = delete; /* Removes new NonCopyable */
};
</pre></div>
<p>This feature is specific to C++ only. The defaulting/deleting is currently ignored, because SWIG
automatically produces wrappers for special constructors and operators specific to the target language.</p>
<H3><a name="Cpp0x_Type_long_long_int"></a>7.2.22 Type long long int</H3>
<p>SWIG correctly parses and uses the new <tt>long long</tt> type already introduced in C99 some time ago.</p>
<H3><a name="Cpp0x_Static_assertions"></a>7.2.23 Static assertions</H3>
<p>SWIG correctly parses and calls the new <tt>static_assert</tt> function.</p>
<div class="code"><pre>
template &lt;typename T&gt;
struct Check {
static_assert(sizeof(int) &lt;= sizeof(T), "not big enough");
};
</pre></div>
<H3><a name="Cpp0x_Allow_sizeof_to_work_on_members_of_classes_without_an_explicit_object"></a>7.2.24 Allow sizeof to work on members of classes without an explicit object</H3>
<p>SWIG correctly calls the sizeof() on types as well as on the
objects. For example:</p>
<div class="code"><pre>
struct A {
int member;
};
const int SIZE = sizeof(A::member); // does not work with C++03. Okay with C++0x
</pre></div>
<p>In Python:</p>
<div class="targetlang"><pre>
&gt;&gt;&gt; SIZE
8
</pre></div>
<H2><a name="Cpp0x_Standard_library_changes"></a>7.3 Standard library changes</H2>
<H3><a name="Cpp0x_Threading_facilities"></a>7.3.1 Threading facilities</H3>
<p>SWIG does not currently wrap or use any of the new threading
classes introduced (thread, mutex, locks, condition variables, task). The main reason is that
SWIG target languages offer their own threading facilities that do not rely on C++.</p>
<H3><a name="Cpp0x_Tuple_types"></a>7.3.2 Tuple types and hash tables</H3>
<p>SWIG does not wrap the new tuple types and the unordered_ container classes yet. Variadic template support is working so it is possible to
include the tuple header file; it is parsed without any problems.</p>
<H3><a name="Cpp0x_Regular_expressions"></a>7.3.3 Regular expressions</H3>
<p>SWIG does not wrap the new C++0x regular expressions classes, because the SWIG target languages use their own facilities for this.</p>
<H3><a name="Cpp0x_General_purpose_smart_pointers"></a>7.3.4 General-purpose smart pointers</H3>
<p>
SWIG provides special smart pointer handling for <tt>std::tr1::shared_ptr</tt> in the same way it has support for <tt>boost::shared_ptr</tt>.
There is no special smart pointer handling available for <tt>std::weak_ptr</tt> and <tt>std::unique_ptr</tt>.
</p>
<H3><a name="Cpp0x_Extensible_random_number_facility"></a>7.3.5 Extensible random number facility</H3>
<p>This feature extends and standardizes the standard library only and does not effect the C++ language and SWIG.</p>
<H3><a name="Cpp0x_Wrapper_reference"></a>7.3.6 Wrapper reference</H3>
<p>The new ref and cref classes are used to instantiate a parameter as a reference of a template function. For example:</p>
<div class="code"><pre>
void f( int &amp;r ) { r++; }
// Template function.
template&lt; class F, class P &gt; void g( F f, P t ) { f(t); }
int main() {
int i = 0 ;
g( f, i ) ; // 'g&lt;void ( int &amp;r ), int&gt;' is instantiated
// then 'i' will not be modified.
cout &lt;&lt; i &lt;&lt; endl ; // Output -&gt; 0
g( f, ref(i) ) ; // 'g&lt;void(int &amp;r),reference_wrapper&lt;int&gt;&gt;' is instantiated
// then 'i' will be modified.
cout &lt;&lt; i &lt;&lt; endl ; // Output -&gt; 1
}
</pre></div>
<p>The ref and cref classes are not wrapped by SWIG because the SWIG target languages do not support referencing.</p>
<H3><a name="Cpp0x_Polymorphous_wrappers_for_function_objects"></a>7.3.7 Polymorphous wrappers for function objects</H3>
<p>SWIG fully supports function template wrappers and function objects:</p>
<div class="code"><pre>
function&lt;int ( int, int )&gt; pF; // function template wrapper
struct Test {
bool operator()( short x, short y ); // function object
};
</pre></div>
<H3><a name="Cpp0x_Type_traits_for_metaprogramming"></a>7.3.8 Type traits for metaprogramming</H3>
<p>The new C++ metaprogramming is useful at compile time and is aimed specifically for C++ development:</p>
<div class="code"><pre>
// First way of operating.
template&lt; bool B &gt; struct algorithm {
template&lt; class T1, class T2 &gt; int do_it( T1&amp;, T2&amp; ) { /*...*/ }
};
// Second way of operating.
template&lt;&gt; struct algorithm&lt;true&gt; {
template&lt; class T1, class T2 &gt; int do_it( T1, T2 ) { /*...*/ }
};
// Instantiating 'elaborate' will automatically instantiate the correct way to operate.
template&lt; class T1, class T2 &gt; int elaborate( T1 A, T2 B ) {
// Use the second way only if 'T1' is an integer and if 'T2' is
// in floating point, otherwise use the first way.
return algorithm&lt; is_integral&lt;T1&gt;::value &amp;&amp; is_floating_point&lt;T2&gt;::value &gt;::do_it( A, B );
}
</pre></div>
<p>SWIG correctly parses the template specialization, template types and values inside the &lt;&gt; block and the new helper functions: is_convertible, is_integral, is_const etc.
However, SWIG still explicitly requires concrete types when using the <tt>%template</tt> directive, so the C++ metaprogramming features are not really of interest at runtime in the target languages.</p>
<H3><a name="Cpp0x_Uniform_method_for_computing_return_type_of_function_objects"></a>7.3.9 Uniform method for computing return type of function objects</H3>
<p>SWIG does not wrap the new result_of class introduced in the &lt;functional&gt; header and map the result_of::type to the concrete type yet. For example:</p>
<div class="code"><pre>
%inline %{
#include &lt;functional&gt;
double square(double x) {
return (x * x);
}
template&lt;class Fun, class Arg&gt;
typename std::result_of&lt;Fun(Arg)&gt;::type test_result_impl(Fun fun, Arg arg) {
return fun(arg);
}
%}
%template(test_result) test_result_impl&lt;double(*)(double), double&gt;;
%constant double (*SQUARE)(double) = square;
</pre></div>
<p>will result in:</p>
<div class="targetlang"><pre>
&gt;&gt;&gt; test_result_impl(SQUARE, 5.0)
&lt;SWIG Object of type 'std::result_of&lt; Fun(Arg) &gt;::type *' at 0x7faf99ed8a50&gt;
</pre></div>
<p>Instead, please use <tt>decltype()</tt> where possible for now.</p>
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