How to have local anonymous C++ function object use argument from containing method?

Question

What's the correct way to have an anonymous local function object access an argument of the containing method? I.e., what's the correct way to do the following:

void A::foo(B& b)
{
    struct {
        void operator()() {b.bar();}
    } func;
    func();
}

NB: This example is contrived for simplicity: the actual use-case involves applying an anonymous local function object to each element in a container to have the element act on the argument of the containing method.

score 5 · Accepted Answer · answered Nov 09 '16 at 00:05

You need to pass the reference/pointer of the worked-on object to the anonymous one. But since it's anonymous, you cannot declare its constructor. If you named it, you'd be able to say:

void A::foo(B& b)
{
    struct Foo{
        B& b;
        Foo(B& b) : b(b) {}
        void operator()() {b.bar();}
    } func{b};
    func();
}

That being said, in C++11 - as you tagged this question - you could use a lambda expression:

void A::foo(B& b)
{
    auto func = [&]{ b.bar(); };
    func();
}

score 2 · Answer 2 · answered Nov 09 '16 at 00:06

A function cannot access stuff from its local scope.

Lambdas get around that by capturing variables in their local scope. They copy/move/reference them. But they're not technically accessing those variables. Only lambdas can do this.

You can of course explicitly do what a lambda does implicitly. That is, have the class's constructor take a copy or reference to a variable, then initialize the object by calling that constructor. Of course, that means it cannot be an anonymous class anymore.

score 0 · Answer 3 · answered Nov 09 '16 at 00:54

In fact you wrote almost all correctly. Here you are

#include <iostream>

struct A
{
    void foo( const struct B &b );
};

struct B
{
    void bar() const { std::cout << "Hello A!" << std::endl; }
};

void A::foo( const B &b )
{
    struct 
    {
        void operator ()( const B &b ) const { b.bar(); }
    } func;

    func( b );
};

int main() 
{
    A().foo( B() );

    return 0;
}

The program output is

Hello A!

A more simpler way to define a functional object is to use lambda expressions.

How to have local anonymous C++ function object use argument from containing method?

3 Answers3