MSVC direct constructor call extension

问题

In this response, tloveless pointed out that it's possible in MSVC to use this->foo::foo(42); for constructor delegation to directly call a constructor:

#include <iostream>

struct foo
{
    int m;
    foo(int p) : m(p) { std::cout << "foo("<<p<<")\n"; }
    foo()
        : m(0)
    {
        this->foo::foo(42);
        std::cout << "foo(), " << m << "\n";
    }
};

int main()
{
    foo f;
    std::cin.ignore();
}

I was surprised that this even compiles in MSVC; clang++, g++ and me agree it's illegal, e.g. [class.ctor]/2 "Because constructors do not have names, they are never found during name lookup"

However, MSVC doesn't even emit a warning with /Wall and without language extensions /Za in MSVC12 Update 1 (2013) and MSVC10 SP1 (2010).

The output is:

foo(42)
foo(), 42

in both versions. So there's no temporary created, but a constructor called.

Questions:

1. What is the name of this extension?
2. Isn't it considered an extension? (/Za and the list of extensions don't seem to think so)
3. Is there some documentation for / official description of this feature?

_{(I tagged this question with the [delegating-constructors] tag since it reminds me heavily of this feature)}

---

meta-info: I'm almost sure this question is a duplicate, since this feature is somewhat known. For example, see this answer to a "similar question". Please do not hesitate closing this as a dup if you can find an answer that describes this feature.

回答1:

It is not constructor delegating. Try following code:

#include <iostream>

class C{
public:
    C() { std::cout << "C" << std::endl; }
    ~C() { std::cout << "~C" << std::endl; }
};

struct foo
{
    int m;
    C c;
    foo(int p) : m(p) { std::cout << "foo("<<p<<")\n"; }
    foo()
        : m(0)
    {
        this->foo::foo(42);
        std::cout << "foo(), " << m << "\n";
    }
};

int main()
{
    foo f;
}

According to output field "c" is initialized twice but destroyed only once. As zneak noted, It is similar to new (this) foo(42).

来源：https://stackoverflow.com/questions/23347437/msvc-direct-constructor-call-extension