Question

Possible Duplicate:
std::string x(x);

class A {};

int main() {

    A a(a);
}

This compiles.

gcc (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2)
g++ -o main main.cpp -Wall -w -ansi

I receive no warnings.

Why does this appear to be valid C++?
Is this mentioned anywhere in the standard?
Are there warning flags that can report this for gcc?

When the class has member data, the data ends up random.
example:

#include <iostream>

class A {

public:
    int i;
    A() : i{6} {}
};

int main() {

    A a(a);
    std::cout << a.i << '\n';
}

output: -482728464

What’s going on here?
Also, how can I prevent myself from accidently doing this? – Is it possible to make it a compiler error?

Answer 1

(§ 3.3.2/1) The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer (if any), except as noted below. [ Example:

int x = 12;
{ int x = x; }

Here the second x is initialized with its own (indeterminate) value. —end example ]

This applies to user-defined types, such as your class A, as well. The copy constructor used is the default one, auto-generated by the compiler.