Capturing a static variable by reference in a C++11 lambda

Question

Main question

I'm trying to compile the following code with GCC 4.7.2:

#include <iostream>  int foo() {     static int bar;     return [&bar] () { return bar++; } (); // lambda capturing by reference }  int main (int argc, char* argv[]) {     std::cout << foo() << std::endl;     return 0; } 

And it seems that is not going well, as the output is this one:

$p2.cpp: In function ‘int foo()’: $p2.cpp:6:14: warning: capture of variable ‘bar’ with non-automatic storage duration [enabled by default] $p2.cpp:4:16: note: ‘int bar’ declared here 

So, my first question would be:

Is this a failure of GCC, or the code is not legit C++11? Is this fixed in any recent version of GCC?

Using the trick in a shared_ptr factory

I consider to build an artifact based on this principle but using a non-literal static variable. This artifact is meant to be a factory of shared_ptr< T > objects, which avoid the creation of new T objects when you just need a duplicate shared_ptr container for the same instance.

This artifact would look like:

std::shared_ptr<Foo> create(std::string name) {     static std::unordered_map<std::string,std::weak_ptr<Foo>> registry;      if (auto it = registry.find(name) != registry.end())         return registry[name].lock();      auto b = std::shared_ptr<Foo>(         new Foo(name),          [&registry] (Foo* p) {             registry.erase(p->getName());             delete p;         });      registry.emplace(name,b);     return b; } 

As far as I know, if the GCC issue discussed before is not a problem in terms of C++11 conformance, this artifact shouldn't be an issue either. The only thing to take care by using this hack, is to not set the resulting shared_ptr< T > object to any global object which could be destructed after the static variable.

Am I right about this?

Answer 1

Why are you even trying to capture bar? It's static. You don't need to capture it at all. Only automatic variables need capturing. Clang throws a hard error on your code, not just a warning. And if you simply remove the &bar from your lambda capture, then the code works perfectly.

#include <iostream>  int foo() {     static int bar;     return [] () { return bar++; } (); // lambda capturing by reference }  int main (int argc, char* argv[]) {     std::cout << foo() << std::endl;     std::cout << foo() << std::endl;     std::cout << foo() << std::endl;     return 0; } 

prints

0 1 2 

Answer 2

Per the standard, you can only capture variables with automatic storage duration (or this, which is mentioned as explicitly capturable).

So, no, you can't do that as per the standard (or, to answer your first question, that is not valid C++ 11 and is not a compiler bug)

5.1.1/2 A name in the lambda-capture shall be in scope in the context of the lambda expression, and shall be this or refer to a local variable or reference with automatic storage duration.

EDIT: And, as Kevin mentioned, you don't even need to capture a local static anyways.