Valid expressions for default function arguments

问题

What are all the possible types of valid expressions for a default argument in a function or member function?

回答1:

Anything that is correct within context of assignment to a variable of function parameter's type.

Edit
The default arguments during compilation are evaluated in terms of type correctness etc, but they are not calculated and no assignment takes place until run-time. You can specify a constructor of a yet to be defined class as a default argument and it's fine, as long as class is defined at the point of function use... The actual calculation/assignment takes place during function call, not at the point of function declaration/definition.

Example:

#include <iostream>

void foo( int a = std::rand())
{
  std::cout << a << std::endl;
}

int main( void )
{
 foo();

 return( 0 );
}

Program output on ideone.com:

1804289383

回答2:

This is detailed in section 8.3.6 of the C++03 standard. It basically amounts to any expression that doesn't depend on anything in local scope, so any expression which depends on local variables, parameters to a function, or on "this" are excluded.

来源：https://stackoverflow.com/questions/9286801/valid-expressions-for-default-function-arguments