auto and sizeof

auto: let C++ choose the type

If the starting value already makes the type obvious, you can write auto and C++ picks the type for you:

#include <iostream>
using namespace std;

int main() {
    auto count = 10;       // C++ sees 10 and picks int
    auto price = 4.5;      // 4.5 has a decimal point, so double
    auto letter = 'z';     // char
    cout << count << " " << price << " " << letter << "\n";
    return 0;
}

Output

10 4.5 z

Tip: auto is handy later, when type names get long. While learning, writing the type yourself (int, double...) helps you understand your code better.

sizeof: how big is a type?

Every variable uses some memory, measured in bytes. sizeof tells you how many:

#include <iostream>
using namespace std;

int main() {
    cout << "char:   " << sizeof(char) << " byte\n";
    cout << "bool:   " << sizeof(bool) << " byte\n";
    cout << "int:    " << sizeof(int) << " bytes\n";
    cout << "double: " << sizeof(double) << " bytes\n";
    return 0;
}

Output

char:   1 byte
bool:   1 byte
int:    4 bytes
double: 8 bytes

An int uses 4 bytes. That is why it can only hold numbers up to about 2.1 billion. Go past that and the number overflows and wraps around to a negative number:

#include <iostream>
using namespace std;

int main() {
    int biggest = 2147483647;
    cout << biggest << "\n";
    cout << biggest + 1 << "\n";     // too big for an int!
    long long safe = 2147483647LL + 1;
    cout << safe << "\n";
    return 0;
}

Output

2147483647
-2147483648
2147483648

Common mistake: Integer overflow is a real source of bugs – and even security holes! If your numbers can get big, use long long.