Function compiling even though it doesn't accept integer [duplicate] - c++

Question

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What is a converting constructor in C++ ? What is it for?
(3 answers)
Closed 3 years ago.
I am confused how can we pass an integer when the parameter of a function only accept a class of type enemy ( void foo(const Enemy& inKlep ).
Yet when we pass to it an int (300) it compiles. Why is this?
#include <iostream>
using namespace std;
class Enemy {
public:
Enemy() { cout << "E ctor" << endl; }
Enemy(int i) { cout << "E ctor " << i << endl; }
Enemy(const Enemy& src) {cout << "E copy ctor"<< endl;}
Enemy& operator=(const Enemy& rhs) {cout<<"E="<<endl;}
virtual ~Enemy() { cout << "E dtor" << endl; }
void hornet(int i=7) const { // Not virtual!
cout << "E::hornet " << i << endl;
}
};
class Scott : public Enemy {
public:
Scott() : Enemy(1) { cout << "S ctor" << endl; }
Scott& operator=(const Scott& rhs) {cout<<"S="<<endl;}
virtual ~Scott() { cout << "S dtor" << endl; }
void hornet(int i=7) const {
cout<<"S::hornet " << i << endl;
}
};
void foo(const Enemy& inKlep) {
Enemy theEnemy;
inKlep.hornet(2);
}
int main(int argc, char** argv) {
foo(300);
cout << "Done!" << endl; // Don't forget me!
}

Answer 1

In C++, it is valid code for an input parameter to implicitly construct an object if the function expects an object that can be constructed from that parameter. So, for example:
struct CustomInt {
int val;
CustomInt() : CustomInt(0) {}
CustomInt(int value) : val(value) {}
};
void func(CustomInt obj) {
std::cout << obj.val << std::endl;
}
int main() {
func(5); //Valid; will print '5' to the console
}
If you don't want to allow this, you need to add the keyword explicit to the constructor to prevent this.
struct CustomInt {
int val;
CustomInt() : CustomInt(0) {}
explicit CustomInt(int value) : val(value) {}
};
void func(CustomInt obj) {
std::cout << obj.val << std::endl;
}
int main() {
//func(5); //Invalid; will cause a compile-time error
func(CustomInt(5)); //Valid; will print '5' to the console
}