Function call destructs return object even when it isn't explicitly constructed? C++

Question

So I call an overloaded addition of two Word objects with

Word w;
w+w;

the declaration and definition are:

Sentence operator+(const Word&) const;

Sentence Word::operator+(const Word& rightWord) const{
std::cout<<"Entering function Word::operator+(const Word&)."<<std::endl;
std::cout<<"Leaving function Word::operator+(const Word&).\n"<<std::endl;
}

After w+w is executed a Sentence object is deconstructed (I overloaded destructor to print to stdout) I created a sentence object earlier, but I don't think that is affecting it. I don't understand how a sentence object is being deconstructed when it wasn't even constructed (overloaded default constructor too). I also don't understand why it would be created since I'm not even actually returning it. I ran it through gdb and it is definitely calling sentence's destructor when it exits the addition function. I can provide more code, just figured someone might know the problem without it.

Your function is declared to return a `Sentence` hence this `Sentence` has to be constructed and destructed at some point — tkausl, Oct 08 '16 at 09:07
I overloaded the default constructor and it wasn't called, only the destructor. And when stepped through it in gdb I never hit a constructor @tkausl — Aiden Cullo, Oct 08 '16 at 09:09
It's just UB, nothing is impossible. See [What happens when a function that returns an object ends without a return statement](http://stackoverflow.com/questions/39118324/what-happens-when-a-function-that-returns-an-object-ends-without-a-return-statem/39118529#39118529) — songyuanyao, Oct 08 '16 at 09:10
Sentence Word::operator+(const Word& rightWord) const ------ That function creates a new word on w+w; and then destructs it cause it wasn't used. — Noam Rodrik, Oct 08 '16 at 09:21
@tkausl: That's not correct. Yes, the function declaration requires the implementation to construct and destruct a `Sentence`, but it does not cause the implementation to do this automatically. The implementation does not meet the requirement, so it's just undefined behaviour. — Christian Hackl, Oct 08 '16 at 10:27

score 0 · Accepted Answer · answered Oct 08 '16 at 10:11

It's undefined behaviour if nothing is returned from a non-void function. All of the effects you have observed are not defined by the C++ language, are specific to your compiler and/or just random.

In fact, your compiler should issue a warning message for this piece of code, or even an error. For example, the following produces error C4716: 'Word::operator+': must return a value with Visual C++ 2015:

#include <iostream>

struct Sentence {};

struct Word {
Sentence operator+(const Word&) const;
};

Sentence Word::operator+(const Word& rightWord) const{
std::cout<<"Entering function Word::operator+(const Word&)."<<std::endl;
std::cout<<"Leaving function Word::operator+(const Word&).\n"<<std::endl;
} // error C4716

int main() {
}

Function call destructs return object even when it isn't explicitly constructed? C++

1 Answers1