CODEWITHSUNDEEP
c++
nabulke
nabulke

Reputation: 11285

Function with missing return value, behavior at runtime

As expected, the compiler (VisualStudio 2008) will give a warning

warning C4715: 'doSomethingWith' : not all control paths return a value

when compiling the following code:

int doSomethingWith(int value)
{
    int returnValue = 3;
    bool condition = false;

    if(condition)
        // returnValue += value; // DOH

    return returnValue;
}

int main(int argc, char* argv[])
{
    int foo = 10;
    int result = doSomethingWith(foo);
    return 0;
}

But the program runs just fine. The return value of function doSomethingWith() is 0.

Is is just undefined behavior, or is there a certain rule how the result value is created/computed at runtime. What happens with non-POD datatypes as return value?

Upvotes: 12

Views: 4056

Answers (4)

alseether
alseether

Reputation: 1993

Updating @piotr answer.

From the C++17 Standard Section 9.6.3

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 (6.6.1) results in undefined behavior.

Upvotes: 3

Muxecoid
Muxecoid

Reputation: 1251

For x86 processors, the standard calling convention puts the return value to the EAX register. Practically it means that for most compilers if we reach the end of the function without returning, the result of the last math operation will be returned. However, you can not rely on it and it is not portable.

http://en.wikipedia.org/wiki/X86_calling_conventions#cdecl

Upvotes: 8

piotr
piotr

Reputation: 5745

It is Undefined behaviour as specified in the ISO C++ standard section 6.6.3:

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

Upvotes: 18

codaddict
codaddict

Reputation: 455440

Not returning a value from a value-returning function leads to undefined behavior.

Upvotes: 2

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