C++ 中布尔函数的默认返回值是多少？

Question

What could be the return value of a boolean function in C++ if I do not return something? And what could be the return value if I just return a true value and not the false one? Here is my code:

bool f(int a)
{
//    return value?
}

bool f(int a)
{
    if(a%2) return true;
//    return value here?
}

Answer 1

The return type is the one you define in the function prototype. Note that there is no default return value for functions in C++ (except for special function main ). A function that is supposed to return a value must return a value in all its code paths. Otherwise you yield undefined behaviour. Cf, for example, this online C++ working draft :

9.6.3 The return statement

...

Flowing off the end of a constructor, a destructor, or a function with a cv void return type is equivalent to a return with no operand. Otherwise, flowing off the end of a function other than main (6.8.3.1) results in undefined behavior.

Answer 2

The behaviour on calling a non- void function (other than main ) where program control reaches a branch that does not have an explicit return value is undefined .

(Note that this is a stronger condition than in C where the behaviour is undefined only if you attempt to use the function return value.)

In C++, you can use auto to get the compiler to work out the return type for you:

auto foo(int n)
{
    if (n >= 0){
        return true;
    } else {
        return false;
    }
}

but the type of foo is still static in the sense that it is known at compile time. This approach is particularly useful when working with templates. It is not designed to allow you to be sloppy with your types.

Answer 3

you can not do that if you write a function that supposes to return something and you return nothing you get and error. so use void or return something.