问题
Is the following code legal in C++?
void f(void* = 0)
{}
int main()
{
f();
}
Which page of the C++ standard states that this usage is legal?
回答1:
Yes, it's legal.
There is no standard wording to allow this combination of features specifically; there simply isn't any to disallow it, either.
Default argument syntax applies to function parameters in a parameter-declaration:
[C++11: 8.3.6/1]:If an initializer-clause is specified in a parameter-declaration this initializer-clause is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.
...and function parameters in a parameter-declaration may be unnamed:
[C++11: 8.3.5/11]:[..] An identifier can optionally be provided as a parameter name. [..]
There is even an example of this usage under 8.3.6/4 (though examples are not normative text, so this cannot be used to prove anything concretely).
回答2:
Yes, it's perfectly legal. An obvious example is found in N3485 8.3.6 Default Arguments/4:
[Example: the declaration
void point(int = 3, int = 4);declares a function that can be called with zero, one, or two arguments of type int.
回答3:
Yes, it is legal.
The syntax productions given for function parameters in clause 8.3.5/1 allow a parameter declaration without an identifier, but with an assignment expression (as initialiser).
回答4:
Not only is it legal, it could actually be quite useful depending on your coding style.
Default parameters are only meaningful in a function declaration.
Named parameters are only meaningful in a function definition.
f.h:
void f(void*=nullptr);
f.cc
void f(void* x)
{
...
}
来源：https://stackoverflow.com/questions/14675908/can-an-unnamed-parameter-of-function-have-a-default-value