将函数的整数返回值与C ++中的bool进行比较

Question

Consider the following C++ code snippet below:

...

if (false == func()) // Line #1
{
 ...
 ...
}

int func()
{
 ...
  {
     ...
     return false;
  }
 ...

 return true;
} 

In the code snippet above, the prototype of function func() indicates that it returns an integer. But inside the function func() , it returns either false or true .

Also in Line #1, the return value of function func() is checked against a boolean value.

I would like to know whether there is any problem with this type of usage. If there is some problem, recommend what rectifications needs to be done.

Answer 1

There is no problem, any non-zero integer translates to true , and zero is false . Conversely, a bool can be converted to int , with true converting to 1 and false to 0 .

You don't need to be so verbose when comparing boolean values though:

if (!func()) { .... }

Of course, if the function only returns true or false , as in your example, then it would make sense for it to return bool directly:

bool func();

Answer 2

From the standard

4.7 Integral conversions

If the destination type is bool, see 4.12. If the source type is bool, the value false is converted to zero and the value true is converted to one.

4.12 Boolean conversions [conv.bool]

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

Answer 3

Inside func , the bool you supply is converted to int for return. false is converted to 0 and true is converted to 1 .

In the comparison, your constant false is converted to int , with value 0 .

Generally your code will read better if you pick one: either use the bool type throughout, or else use the int type and the value 0 .

Answer 4

In the function func(), the source type is bool and the destination type is int (since the return type of the function is int), so the value false is converted to zero and the value true is converted to the value one of the destination type. This int, when returning back to Line #1 is automatically converted to bool so, since func can return 1 or 0, the value zero becomes false and 1 becomes true. If func returned any non-zero value, it would be automatically converted to true on Line #1.

Check this website in the sections "Integral conversions" and "Boolean conversions".

Sureley there's no problem with this type of usage but I must say that it's not easily readable, expecially if the code becomes more complex and should be maintained by someone else that didn't write this code. Why don't you simply make func return bool?