C string to uppercase in C and C++

Question

While I was putting together a to-uppercase function in C++ I noticed that I did not receive the expected output in C.

C++ function

#include <iostream>
#include <cctype>
#include <cstdio>

void strupp(char* beg)
{
    while (*beg++ = std::toupper(*beg));
}

int main(int charc, char* argv[])
{
    char a[] = "foobar";
    strupp(a);
    printf("%s\n", a);
    return 0;
}

Output as expected:

FOOBAR

---

C function

#include <ctype.h>
#include <stdio.h>
#include <string.h>

void strupp(char* beg)
{
    while (*beg++ = toupper(*beg));
}

int main(int charc, char* argv[])
{
    char a[] = "foobar";
    strupp(a);
    printf("%s\n", a);
    return 0;
}

The output is the expected result with the first character missing

OOBAR

Does anyone know why the result gets truncated while compiling in C?

Answer 1

The problem is that there is no sequence point in

while (*beg++ = toupper(*beg));

So we have undefined behavior. What the compiler is doing in this case is evaluating beg++ before toupper(*beg) In C where in C++ it is doing it the other way.