Is there a difference between foo(void) and foo() in C++ or C?

Consider these two function definitions:

void foo() { }

void foo(void) { }

Is there a difference between these two? If not, why is the void argument there? Is it for aesthetic reasons?

Historical note: this answer applies to C17 and older editions. C23 and later editions treat void foo() differently.

In C:

• void foo() means "a function foo taking an unspecified number of arguments of unspecified type"

• void foo(void) means "a function foo taking no arguments"

In C++:

• void foo() means "a function foo taking no arguments"

• void foo(void) means "a function foo taking no arguments"

By writing foo(void), therefore, we achieve the same interpretation across both languages and make our headers multilingual (though we usually need to do some more things to the headers to make them truly cross-language; namely, wrap them in an extern "C" if we're compiling C++).

Stack Overflow Q#51032, score: 365