I have an int function that searches an array for a value, and if the value is found it returns the value of the position in the array. If the value isn't found, I simply put
return false;
Would that give me the same result as return 0;?
I would also like to know what exactly would happen if I did return NULL;.
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2 Answers
Unlike PHP and other languages, C++ types aren't changed on the fly. A way to do what you want (return false if something didn't work but return an int if it did) would be to define the function with a return type of bool but also pass an int reference (int&) in it. You return true or false and assign the reference to the correct value. Then, in the caller, you see if true was returned and then use the value.
bool DoSomething(int input, int& output)
{
//Calculations here
if(/*successful*/)
{
output = value;
return true;
}
output = 0; //any value really
return false;
}
// elsewhere
int x = 5;
int result = 0;
if(DoSomething(x, result))
{
std::cout << "The value is " << result << std::endl;
}
else
{
std::cout << "Something went wrong" << std::endl;
}
Comments
false will be converted to 0 from the draft C++ standard section 4.7 Integral conversions paragraph 4 which says (emphasis mine going foward):
[...]If the source type is bool, the value false is converted to zero and the value true is converted to one.
I think most people would find that odd but it is valid. As for NULL section 4.10 Pointer conversions says:
A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t. [...]
5 Comments
Chris
That's what I had suspected, I was just hoping it would work like a boolean because I used the actual word false. I'll just use a different method. Thank you. NULL would just go to 0 also? Well then. Thank you for the very complete answer.
Shafik Yaghmour
@ChrisVachon well
0 will convert to false and non-zero to true, not totally sure what you are trying to do though.Chris
I'm actually trying to go the other way around. I'm supposed to return an int but return the value false.
mangusta
is it actually possible to return a data type that is different from return type declared in function prototype in c++ ? it is possible in c (in turboC at least)
boB
Using false or NULL will return 0 ... HOWEVER there is no way to tell either of those translated zeros from an actual zero result. if zero is a valid legitimate return value, then you may be getting a false sense of security from using keywords NULL or false.