(Known) compiler bug in VC12?

问题:

This program, when compiled with VC12 (in Visual Studio 2013 RTM)^[1] leads to a crash (in all build configurations), when really it shouldn't:

#include <string>

void foo(std::string const& oops = {})
{
}

int main()
{
    foo();
}

I know of two silent bad codegen bugs that might be related:

• https://connect.microsoft.com/VisualStudio/feedback/details/800364/initializer-list-calls-object-destructor-twice
• http://connect.microsoft.com/VisualStudio/feedback/details/800104/

Honestly I think these are different, though. Does anyone know

1. whether there is an actively tracked bug on connect for this
2. whether there is a workaround (or an explicit description of the situation that causes this bug, so we can look for it/avoid it in our code base)?

---

^[1] Just create an empty project using the C++ Console Application 'wizard'. For simplicity, disable precompiled headers and leave all defaults: http://i.stack.imgur.com/rrrnV.png

回答1:

An active issue was posted back in November. The sample code posted was:

Compile and run following code in VS2013

#include <string>

void f(std::string s = {}) {
}

int main(int argc, char* argv[]) {
    f();
    return 0;
}

The bug has been acknowledged by Microsoft.

There doesn't seem to be a work-around posted there. Edit Workarounds can easily be based on avoiding the list-initializer syntax:

void f(std::string s = "");
void f(std::string s = std::string());
void f(std::string s = std::string {});

Or just the old-fashioned (if you don't mind introducing overloads):

void f(std::string s);
void f() { f(std::string()); }

回答2:

It looks like Visual Studio is just broken with respect to which constructor it calls when the default argument is an initializer list. This code:

#include <iostream>

struct test {
  test ()  { std::cout << "test ()" << std::endl ; } 
  test (int)  { std::cout << "test (int)" << std::endl ; }
};

void func( test const &s = {} )
{
}

int main()
{
    test s = {} ;
    func() ;
}

produces this result in gcc and clang, see it live here:

test ()
test ()

while Visual Studio produces this result:

test ()
test (int)

and for this code:

#include <iostream>
#include <initializer_list>

struct test {
  test ()  { std::cout << "test ()" << std::endl ; };

  test (int)  { std::cout << "test (int)" << std::endl ; };
  test ( std::initializer_list<int>) { std::cout << "test (initializer_list<int>)" << std::endl ; } ;
};

void func( test const &s = {0} )
{
}

int main()
{
    test s = {0} ;
    func() ;
}

gcc and clang produce this result see it live here:

 test (initializer_list<int>)
 test (initializer_list<int>)

while Visual Studio produces this error:

 error C2440: 'default argument' : cannot convert from 'initializer-list' to 'const test &'
    Reason: cannot convert from 'initializer-list' to 'const test'
    No constructor could take the source type, or constructor overload resolution was ambiguous

Update

For a sanity check, I went back to the standard to make sure there wasn't some odd rule at the root of this difference or perhaps some restriction that makes this code ill-formed. As far as I can tell this code is not ill-formed. Section 8.3.5 grammar specifically allows this:

parameter-declaration:
  attribute-specifier-seqopt decl-specifier-seq declarator
  attribute-specifier-seqopt decl-specifier-seq declarator = initializer-clause
  [...]

It does not seem like section 8.5 Initializers or 8.3.6 Default arguments add any restrictions but this defect report 994. braced-init-list as a default argument and working paper Wording for brace-initializers as default arguments make it clear that it was intended and outline the changes made to the standard to allow it and looking at the deltas there are no obvious restrictions.