printf函数中的点和const

Question

I wrote the following code in c++:

int printf(const char *p,...);
int main()
{
    printf("Stack Overflow\n");
    return 0;
}

It returned an error :

/home/tWi2Su/ccnhXznj.o: In function `main':
prog.cpp:(.text.startup+0x17): undefined reference to `printf(char const*, ...)'
collect2: error: ld returned 1 exit status

But when I remove the const , I get the following error :

prog.cpp: In function 'int main()':
prog.cpp:4:27: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
  printf("Stack Overflow\n");
                           ^
/home/a5vnrT/cclz99Yy.o: In function `main':
prog.cpp:(.text.startup+0x17): undefined reference to `printf(char*, ...)'
collect2: error: ld returned 1 exit status

If I remove the dots , I get the following error :

/home/3jQpK8/cc01lRrz.o: In function `main':
prog.cpp:(.text.startup+0x17): undefined reference to `printf(char const*)'
collect2: error: ld returned 1 exit status

Compilation error   time: 0 memory: 0 signal:0

prog.cpp: In function 'int main()':
prog.cpp:4:27: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
  printf("Stack Overflow\n");
                           ^
/home/a5vnrT/cclz99Yy.o: In function `main':
prog.cpp:(.text.startup+0x17): undefined reference to `printf(char*, ...)'
collect2: error: ld returned 1 exit status

Why is it necessary to give dots here when I am the one declaring the function (and not predifining it). Moreover when we pass a string to a function in c++, we do not need to mention const . Then why is necessary to give the const keyword here?

Answer 1

In your case,

int printf(const char *p, ...);
int main()
{
    printf("Stack Overflow\n");
    return 0;
}

you only have a declaration , no definition of printf() . Linker is shouting.

Regarding the (missing) const type-qualifier, in C++ , a string literal, (narrow string literals) has a type “array of n const char ”, and when you try to pass that to a char * , type mismatch happens.

That said, those dots, ... are used as the notation of a variadic function which can accept variable number of arguments.