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.

Answer 2

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

leads to undefined behavior.

Whether *beg++ is sequenced before or after std::toupper(*beg) is unspecified.

The simple fix is to use:

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