Why does the compiler find my function if is not yet declared?

问题

Contrary to my expectations, this program works:

#include <iostream>


namespace a { struct item{}; }
namespace b { struct item{}; }


template<typename T>
void func(T t) { do_func(t); }


int main()
{    
    func(a::item{});
    func(b::item{});
}


namespace a { void do_func(item) { std::cout << "a::func\n"; } }
namespace b { void do_func(item) { std::cout << "b::func\n"; } }

Output:

a::func
b::func

Verifications with online compilers:

gcc 4.8
clang 3.4

If the instantation of func<T> occurs in the body of main then I would expect that a::do_func and b::do_func are not yet declared.

How can this work?

Update

According to @Marc Claesen the reason that above works is:

template instantiation is performed after reading all of the source

However, then why does this code does not work:

#include <iostream>

template<typename T>
void func(T t) { do_func(t); }

int main()
{
    func(1);
}

void do_func(int) { std::cout << "do_func(int)\n"; }

See gcc-4.8:

error: 'do_func' was not declared in this scope,
and no declarations were found by argument-dependent
lookup at the point of instantiation [-fpermissive]

clang++ 3.4:

error: call to function 'do_func' that is neither
visible in the template definition nor found by
argument-dependent lookup

So it seems that the combination of function template and ADL are required to make it work.

However, I don't understand why this is so..

回答1:

It works because of two interesting things:

two-phase name lookup which is performed to lookup dependent names.
and Argument Dependent Lookup (ADL).

Have a look at this:

The Dreaded Two-Phase Name Lookup

In short, do_func is a dependent name, so in the first phase (when the file is only parsed but the function template is not instantiated) the compiler does not resolve the name do_func, it only checks the syntax and it sees it is a valid function call. That is all. In the second phase when the function template is instantiated (and thus T is known), the name do_func is resolved and at this time it also uses ADL to lookup the name.

Note that ADL works only for user-defined types. It doesn't work for built-in types, which is why your second code (i.e func(1)) doesn't work!

来源：https://stackoverflow.com/questions/16623408/why-does-the-compiler-find-my-function-if-is-not-yet-declared

标签

c++

overload-resolution