Please explain what is a void? And how to use it? Please

Answer 1

"Void is nothingness. The truth of life, the external materialisation of the heart."
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Yes, void is indeed nothingness. In C++, void functions are functions which do not return anything back to the caller. (functions which have no return value) This may seem to be redundant at first, but void functions have a lot of usages. E.g.
void hello()
{
std::cout << "Hello!";
}
This is a void function which sole purpose is to print a string literal. No return value is required. Variables can also be passed to the void function via reference to be manipulated. E.g.
void swap(int& a, int& b)
{
a = a + b;
b = a - b;
a = a - b;
// variables manipulated
// no return value required
}

Answer 2

then there is void pointers
int value = 1;
string another = "boo";
void* ptr;
ptr = &value;
ptr = &another;
http://www.learncpp.com/cpp-tutorial/613-void-pointers/