Call of overloaded function is ambiguous

Question

What does this error message mean?

error: call of overloaded ‘setval(int)’ is ambiguous huge.cpp:18: note: candidates are: void huge::setval(unsigned int) huge.cpp:28: note:                 void huge::setval(const char*) 

My code looks like this:

#include <iostream> #define BYTES 8 using namespace std ;  class huge { private:     unsigned char data[BYTES]; public:     void setval(unsigned int);     void setval(const char *);   };  void huge::setval(unsigned int t) {     for(int i = 0; i< BYTES ; i++) {         data[i] = t;         t = t >> 1;     } }  void huge::setval(const char *s) {     for(int i = 0; i< BYTES ; i++)         data[i] = s[i]; }  int main() {     huge p;     p.setval(0);     return 0; } 

Answer 1

The literal 0 has two meanings in C++.
On the one hand, it is an integer with the value 0.
On the other hand, it is a null-pointer constant.

As your setval function can accept either an int or a char*, the compiler can not decide which overload you meant.

The easiest solution is to just cast the 0 to the right type.
Another option is to ensure the int overload is preferred, for example by making the other one a template:

class huge {  private:   unsigned char data[BYTES];  public:   void setval(unsigned int);   template <class T> void setval(const T *); // not implemented   template <> void setval(const char*); }; 

Answer 2

The solution is very simple if we consider the type of the constant value, which should be "unsigned int" instead of "int".

Instead of:

setval(0) 

Use:

setval(0u) 

The suffix "u" tell the compiler this is a unsigned integer. Then, no conversion would be needed, and the call will be unambiguous.