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Slightly better decltype() support for expressions

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decltype now accepts C++ expressions instead of just an ID, such as:

  int i,j;
  ...  decltype(i+j) ...
  ...  decltype(&i) ...

These result in a warning for non-trivial expressions which SWIG cannot evaluate:

  Warning 344: Unable to deduce decltype for 'i+j'.

See 'Type Inference' in CPlusPlus.html for workarounds.

Issue #1589
Issue #1590
This commit is contained in:
William S Fulton 2022-11-26 01:16:20 +00:00
commit 2a1711e436
8 changed files with 85 additions and 14 deletions
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35
Doc/Manual/CPlusPlus11.html
View file

@ -720,18 +720,45 @@ AltStruct var2{2, 4.3}; // calls the constructor
<H3><a name="CPlusPlus11_type_inference">7.2.6 Type inference</a></H3>
<p>SWIG supports <tt>decltype()</tt> with some limitations. Single
variables are allowed, however, expressions are not supported yet. For
<p>SWIG supports <tt>decltype()</tt> with limitations. Single
variables are allowed, however, non-trivial expressions are not supported very well. 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>
<p>SWIG is able to deduce that the variable, <tt>i</tt>, is type <tt>int</tt>.
</p>
<p>
Using a non-trivial expression for the decltype results in a warning:</p>
<div class="code"><pre>
int i; int j;
decltype(i+j) k; // syntax error
decltype(i+j) k; // Warning 344: Unable to deduce decltype for 'i+j'.
</pre></div>
<p>
This warning should be viewed as a prompt to add in a manual ignore of the variable/function as
in most cases the generated code will not compile.
For the example above, ignore the symbol that uses decltype and perhaps additionally
suppress the warning as follows:
</p>
<div class="code"><pre>
#pragma SWIG nowarn=SWIGWARN_CPP11_DECLTYPE
%ignore k;
</pre></div>
<p>
If an ignore is not acceptable, a workaround is to redefine the symbol with the actual type, for example:
</p>
<div class="code"><pre>
int k; // define k with the actual type
%ignore k; // ignore the real definition of k
int i; int j;
decltype(i+j) k; // Warning 344: Unable to deduce decltype for 'i+j'.
</pre></div>
<p>SWIG does not support <tt>auto</tt> as a type specifier for variables, only

3
Doc/Manual/Warnings.html
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@ -438,6 +438,9 @@ example.i(4) : Syntax error in input(1).
<li>325. Nested <em>kind</em> not currently supported (<em>name</em> ignored).
<li>326. Deprecated %extend name used - the <em>kind</em> name '<em>name</em>' should be used instead of the typedef name '<em>name</em>'.
<li>327. Extern template ignored.
<li>340. Lambda expressions and closures are not fully supported yet.
<li>343. Only the first variadic template argument is currently supported.
<li>344. Unable to deduce decltype for '<em>expr</em>'.
<li>350. operator new ignored.
<li>351. operator delete ignored.
<li>352. operator+ ignored.