The only function in C++ which should not have a function prototype is main. This is because it is illegal for another function to call main within a C++ program.

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ANSI/ISO/IEC 9899:1990 International Standard For C
The function called at program startup is named main. The implementation declares no prototype for this function. It can be defined with no parameters:  
int main(void) { /* ... */ }  
or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):
 
int main(int argc, char *argv[ ]) { /* ... */ }  
The newly ratified update to the C standard in 1999 will make this even clearer, perhaps because of all the clueless who could not understand that only the two formats above, both of which define main to return an int, are valid.  
The draft of the new standard expands on the two definitions above by modifying the wording "It can be defined" to "It shall be defined with a return type of int".
 
ANSI/ISO/IEC 14882:1998 International Standard For C++
An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a return type of type int but otherwise its type is implementation defined. All implementations shall allow both of the following definitions of main:  
The two definitions which follow are identical to those in the C standard