Accidential use of classes inside of c style typeless variable arguments list is a

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Asked: May 16, 20262026-05-16T05:53:58+00:00 2026-05-16T05:53:58+00:00

Accidential use of classes inside of c style typeless variable arguments list is a

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Accidential use of classes inside of c style typeless variable arguments list is a common error source.
Example:

class MyString {
    public:
    char *pChars;
    int Length;

    MyString(char *pChars) {
        this->pChars = pChars; 
        Length = strlen(pChars); 
        } };

int main() {
    MyString s1("Bla1"), s2("Bla2");
    printf("%s%s", s1, s2); // This does not but should give a compiler warning/error!
    return 0; }

The printf call there receives the two s objects by value. that means all of their members are simply memory copied. But they are interpreted a simple char pointers. Result is a runtime error of course.

I am not asking for a solution to this, but I would like to have something I could add to my class so that the compiler warns me about it or gives an error.

Already tried to declarate but not implement a copy constructor. But it seems that no copy constructor is called. 🙁

Please just answer to the question in the title. I do not need a discusson of why you should not use printf or these variable arguments lists – know that.

Thanks for your time.