CPP_explicit_keyword_explained.cpp

// In C++, a constructor with only one required parameter is considered an implicit conversion function.
// It converts the parameter type to the class type. 
// Whether this is a good thing or not depends on the semantics of the constructor.

#include <iostream>
#include <string>

using namespace std;

class Foo
{
public:
	// single parameter constructor, can be used as an implicit conversion
	explicit Foo(int foo) : m_foo(foo)
	{
		cout << "int Foo object constructed with foo input " << foo << endl;
	}

	// Can be auto constructed by compiler at compile time 
	// (works only for single argument constructors)
	Foo(string foo) : s_foo(foo)
	{
		cout << "string Foo object constructed with foo input " << foo << endl;
	}

	int GetFoo() { return m_foo; }

private:
	int m_foo;
	string s_foo;
};

// Here's a simple function that takes a Foo object:
void DoBar(Foo foo)
{
	int i = foo.GetFoo();
}

// and here's where the DoBar function is called.
int main()
{
	DoBar(42); // Compiler error because of 'explicit' keyword
	DoBar(string("A")); // No probs
}