template sum function freezes for char

Question

#include <iostream>  // std::cout

template <class T>
T Sum(T x, T y) {
  return x + y;
}
int main() {
  std::cout << Sum<int>(5, 10);
  std::cout << "\n" << 'a' + 'b';
  std::cout << "\n" << Sum<char>('a', 'b');
  return 0;
}

The program freezes on the last line when using cpp.sh site, can somebody please explain why?

which compiler? Works for [clang and gcc](https://godbolt.org/z/9jpHpR). It there is no reason to freeze. Works on [msvc too](https://rextester.com/LBIQ45755). — Marek R, May 22 '20 at 11:18
The compiler can deduce the type. You may use `Sum('a', 'b')`. — user2672165, May 22 '20 at 11:20
To be clear by "it works" I meant it doesn't freeze as OP describes the problem. — Marek R, May 22 '20 at 11:25
@MarekR I checked in online C++ shell http://cpp.sh/ and g++ — DoehJohn, May 22 '20 at 11:26
@n.'pronouns'm. same as the second line. Second line outputs 195 — DoehJohn, May 22 '20 at 11:27
@WernerHenze programs runs and never terminates. In online shell http://cpp.sh/ it's showing "rinning" — DoehJohn, May 22 '20 at 11:28
@Mar "not your code" let's be clear, the code likely has undefined behavior. — Aykhan Hagverdili, May 22 '20 at 11:28
@DoehJohn this is bug of this site. Most probably it expects UTF-8 encoding in output and your program prints a character which says there will be continuation of UTF-8 encoded character, but nothing else is printed. — Marek R, May 22 '20 at 11:30
The second line outputs an integer because `'a'+'b'` has type `int`. `Sum('a', 'b')` has type `char`. These are different things and you cannot expect them to work the same. — n. m. could be an AI, May 22 '20 at 11:37

score 2 · Accepted Answer · answered May 22 '20 at 11:19

2

Assuming signed 8 bit char, which is very common, adding 'a' and 'b' and storing the result in a char overflows and causes undefined behavior. Thus, the program is free to do anything. In my testing with GCC it prints some garbage and main exits with 0.

answered May 22 '20 at 11:19

Aykhan Hagverdili

28,141
6
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93

StefanKssmr · Answer 2 · 2020-05-22T11:31:17.970

0

As @Ayxan already pointed out, adding two chars and storing the result in a char is not a good idea, you can change the return type of your function to auto as the operationchar + char returns int (see this answer). Simply write

template <class T> auto Sum(T x, T y)
{
    std::cout << sizeof(x) << " " << sizeof(y) << " " << sizeof(x+y) << std::endl;;
    return x + y;
}

Running your main will output:

See the live demo.

edited May 22 '20 at 11:31

answered May 22 '20 at 11:21

StefanKssmr

1,196
9
16

he written that application hangs on last line. So this is not the problem. – Marek R May 22 '20 at 11:24
@MarekR You're right, I haven't read carefully enough that he was only looking for an explanation and not a fix. I'll still leave this here, maybe it helps someone else. – StefanKssmr May 22 '20 at 11:37

score 0 · Answer 3 · answered May 22 '20 at 12:01

Problem is site you used for testing. As I showed in comment your code doesn't hangs on:

godbolt with gcc or clang
rextester.com with msvc

Problem is on site cpp.sh site.

Most probably this site is configured to assume UTF-8 encoding in application output. Now when you do addition of char characters are promoted to int that is why 195 is printed in earlier line. Your Sum clips this results to char type (here is undefined behavior since there is integer overflow).

Now in UTF-8 this byte 195 represents is an indicator that current character is build from two bytes. Your program never sends next byte. This site should be able to handle malformed UTF-8 and noticed that stream has ended and not wait for next character. It waits for this character and as a result it hangs. So this is a bug of site cpp.sh.

template sum function freezes for char

3 Answers3