Expected behaviour with c++ lambdas and static variables

Question

I'm using VS2013, and discovered what appears to me to be strange behaviour when using multiple instances of a class that contains lambdas, and those lambdas contain static variables. The static variables appear to be shared.

Example code, very much trimmed down but still captures the essence:

class HasLambda
{
public:
    typedef const char* ( *ToCharPtr ) ( const int& );
    void Init( ToCharPtr pfnToCharPtr ) {
        m_pfnCharPtrConverter = pfnToCharPtr;
    }

    const char* IntToString( int i ) {
        return m_pfnCharPtrConverter( i );
    }

    static HasLambda* Make() {
        HasLambda* pHasLambda = new HasLambda;
        pHasLambda->Init( [] ( const int &i ) -> const char* { static char buf[ 33 ]; sprintf( buf, "%d", i ); return buf; } );
        return pHasLambda;
    }

protected:
    ToCharPtr m_pfnCharPtrConverter;
};

int _tmain(int argc, _TCHAR* argv[])
{
    HasLambda* a;
    a = HasLambda::Make();

    HasLambda* b;
    b = HasLambda::Make();

    const char* aValue = a->IntToString( 7 );
    printf( "a: %s\n", aValue );

    const char* bValue = b->IntToString( 42 );
    printf( "b: %s\n", bValue );
    printf( "a: %s\n", aValue );

    return 0;
}

The output I get is:

a: 7
b: 42
a: 42

I would have expected the second a: value to be the same as the first. Am I seeing a compiler bug, or am I misunderstanding the way lambdas and static variables therein work? Am I using the lambda wrong in some way?

Answer 1

A lambda is not an object that is created when needed, but a shorthand for an inline definition of a class. Your invocation above would be roughly equivalent to:

class SomeLambda {
 public:
  const char* operator() (const int& i) {
    static char buf[33];
    sprintf(buf, "%d", i);
    return buf;
  }
};

...
pHasLambda->Init(SomeLambda());

The static initialization rules there have the same meaning as any function level static for a member function.

If you instead had two different lines creating the lambda ex:

auto x = []() { static char buf[99]; use_buf(buf); return buf; };
auto y = []() { static char buf[99]; use_buf(buf); return buf; };

Then x and y would be separate classes despite having identical definitions.