Ambiguity in c++ constructor when a constructor with default argument exists

问题

 #include<iostream>
 using namespace std;
 class abc{
    int a;
   public: abc() { } //do nothing constructor
           abc(int x=6){ a=x;} //constructor with default argument  
  };
 main()
  {
      abc a;
    ....
  }

my question is which constructor will be invoked in this case ? Please explain

回答1:

This will not compile due to the ambiguity as you can see here

prog.cpp:8:7: warning: ISO C++ forbids declaration of ‘main’ with no type [-Wreturn-type]
prog.cpp: In function ‘int main()’:
prog.cpp:10:11: error: call of overloaded ‘abc()’ is ambiguous
prog.cpp:10:11: note: candidates are:
prog.cpp:6:12: note: abc::abc(int)
prog.cpp:5:12: note: abc::abc()

回答2:

The call you are making to the constructor is ambiguous. The compiler cannot know which constructor to call since one of your constructor contains a default argument.

Instead, try removing the default value to the argument.

来源：https://stackoverflow.com/questions/17770113/ambiguity-in-c-constructor-when-a-constructor-with-default-argument-exists