C++ nested lambda bug in VS2010 with lambda parameter capture?

7
votes

I'm using Visual Studio 2010, which apparently has some buggy behavior on lambdas, and have this nested lambda, where the inner lambda returns a second lambda wrapped as a std::function (cf. "Higher-order Lambda Functions" on MSDN):

int x = 0;
auto lambda = [&]( int n ) 
{ 
    return std::function<void()>( 
        [&] // Note capture
        { 
            x = n; 
        } 
    ); 
};

lambda( -10 )(); // Call outer and inner lambdas

assert( -10 == x ); // Fails!

This compiles but fails at the assert. Specifically, n in the inner lambda is uninitialized (0xCCCCCCCC), but x is successfully modified to its value. If I change the inner lambda's capture clause to "[&,n]", the assert passes as expected. Is this a bug with VS2010 or have I not understood how lambda capture works?

2
c++visual-studio-2010lambdac++11

2 Answers

8
votes

It is not a bug, since n goes out of scope after lambdas return statement, thus the capture by reference is invalidated by the time you use it.

int x = 0;
auto lambda = [&]( int n ) 
{ 
    return std::function<void()>( // n is local to "lambda" and is destroyed after return statement, thus when you call the std::function, the reference capture of n is invalid.
        [&]
        { 
            x = n; // Undefined behaviour
        } 
    ); 
};

auto tmp = lambda(-10); 
// n is no longer valid
tmp(); // calling tmp which uses reference of n which is alrdy destroyed.

assert( -10 == x ); // Fails!
3
votes

This is similar to the case of just returning a simple reference. The thing that caught you was the compiler did not issue an warning. So it is not a bug in the compiler, it is just a lack of a warning.

std::function<int()> F(int n)
{
    return [&]{ return n; };  //no warning
}
int& F2(int n)
{
    return n; //warning
}