What is the difference between this:

Question

#include <iostream>

int main() {
int b = 2;
double a = b;
std::cout << a;
return 0;
}

and this:

#include <iostream>

int main() {
int b = 2;
double a = (double)b;
std::cout << a;
return 0;
}

Answer 1

In the first program, the type casting is done implicitely - by the compiler - while in the second example, it is explicit.

General:
https://www.quora.com/What-is-the-difference-between-the-implicit-type-casting-and-explicit-type-casting-in-C++
Differences:
https://stackoverflow.com/questions/7099957/implicit-vs-explicit-conversion
C++:
http://www.cplusplus.com/doc/tutorial/typecasting/

Answer 2

there isn't difference in the result, the explicit (double) conversion is not necessary.

This is known as a standard conversion. Standard conversions affect fundamental data types, and allow the conversions between numerical types (short to int, int to float, double to int...),