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?
If no return statement appears in a function definition, control automatically returns to the calling function after the last statement of the called function is executed. In this case, the return value of the called function is undefined.
If control reaches the closing curly brace ( } ) of a non- void function without evaluating a return statement, using the return value of the function call is undefined behavior.
nothing, the return value gets ignored.
It means that there is no default return value for your function outside of the for loop.
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.
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.
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
Not returning a value from a value-returning function leads to undefined behavior.
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