Question

Here’s the code.

#include<struct.h>
#include<iostream>
#include<functional>
using namespace std;

void LambdaTest(const function <struct dummy (void)>& f)
    {
    struct dummy test = f();
    cout<<test.a<<endl;
    }

int main()
    {
    int val = 5;
    struct dummy dum;

    auto func = [val](void) -> struct dummy
                        {
                        dummy temp;
                        temp.a = val;
                        return temp;
                        };

    LambdaTest(func);
    return 0;
    }

The file struct.h is very simple.

struct dummy
    {
    int a;
    };

GCC complains that

lambda_struct.cpp:19:38: error: field ‘temp’ has incomplete type

Is this allowed? If yes, then how do I fix it? If not, then why not?

EDIT:

The return type bug in the code (discovered by others) has now been fixed.

SOLUTION:

The problem is that C++0x standard allows definition to a new struct (and a class too, presumably) in the return type of a lambda definition itself. So if struct keyword is present in the return type, the compiler will think that it is a new type and begin to complain.

The fixed code is

#include<struct.h>
#include<iostream>
#include<functional>
using namespace std;

void LambdaTest(const function <struct dummy (void)>& f)
    {
    struct dummy test = f();
    cout<<test.a<<endl;
    }

int main()
    {
    int val = 5;
    struct dummy dum;

    auto func = [val](void) -> dummy
                        {
                        dummy temp;
                        temp.a = val;
                        return temp;
                        };

    LambdaTest(func);
    return 0;
    }

Answer 1

The problem is that GCC incorrectly thinks you’re declaring a new struct type on the trailing return, and it declares a field of an incomplete type that is the same type GCC thinks you’re declaring.

It also complains that

error: ‘temp’ does not name a type

on the line with the assignment, because it is expecting a member declaration, not a statement.

Changing to:

auto func = [val](void) -> dummy
                        {
                            struct dummy temp;
                            temp.a = val;
                            return temp;
                        };

Will work.

Also, beware that not returning a value from a function will probably lead you into the realm of undefined behaviour.