Why is not the compiler giving error? [duplicate] - c++

Question

This question already has answers here:
Warnings or errors for C++ implicit conversion of primitives
(6 answers)
Closed 5 years ago.
I am using the following sample program in C++
#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;
namespace mine{
template<class T>
inline void swap(T &a, T &b){
char c= a; //This should not have compiled
a=b;
b=c;
}
}
int main(){
int a,b;
cout<< "Enter two values: ";
cin>>a>>b;
mine::swap(a,b); //type variable T is instantiated as in
cout << a <<' '<<b << endl;
}
I am expecting the compiler to throw an error in the swap function, because c is declared as a char, but assigned a variable of generic type variable T. Not only that, when invoking swap, T is instantiated as int. However, not only is g++ not giving any error, the program works perfectly. Why is this the case?

Answer 1

C++ gives you the ability to shoot yourself in the foot.
The fact remains that any integral type is convertible to a char with implementation defined behaviour.
The compiler is assuming you know what you are doing, that's all.
auto c = a; would be the best replacement these days. Prior to C++11 you could have written T C = a; (you still can of course.) Although since C++11 you ought to use std::move when swapping, see how std::swap is implemented on your platform. (Reference How does the standard library implement std::swap?)
gcc will warn you of this if you specify -Wconversion on the command line.

Answer 2

First of all, converting int to char is legal, it is a narrowing conversion, and the compiler might warn you if you configure it to do so.
As for why the code compiles, this is due to the fact that the type is known at compile time, therefore the compiler knows that for all the instances of T that it initialised, T is convertible to char.
If you will change a to a type that is not convertible to char, the compiler will complain: e.g with MSVC
The following code give error C2440: 'initializing' : cannot convert from 'std::string' to 'char':
#include "stdafx.h"
#include<iostream>
#include<ctime>
#include<cstdlib>
using namespace std;
namespace mine{
template<class T>
inline void swap(T &a, T &b){
char c= a; //This should not have compiled
a=b;
b=c;
}
}
int _tmain(int argc, _TCHAR* argv[])
{
string a("test");
string b("test2");
mine::swap(a,b); //type variable T is instantiated as in
}