Variables, types and constants

Why types?

A variable is a name for a piece of memory that holds a value. In C++ every variable has a type, and the type never changes. The type tells the compiler how many bytes to use and which operations make sense. C++ is statically typed: types are checked at compile time, so many mistakes are caught before the program ever runs.

The basic types

Sizes are "typical" because the C++ standard only sets minimums; on all common desktop compilers today int is 4 bytes and long long is 8. sizeof(type) tells you the size in bytes on your machine.

#include <iostream>
#include <string>

int main() {
    int seats = 72;
    double fare = 1450.5;
    char coach = 'B';
    bool confirmed = true;
    std::string name = "Asha";

    std::cout << name << " coach " << coach << ", seat " << seats
              << ", fare " << fare << ", confirmed " << confirmed << '\n';
}

Always initialise

Declaring a variable means creating it: int x;. Initialising means giving it a first value: int x = 5;. A local int that is declared but not initialised holds garbage: whatever bits were in that memory. Reading it is undefined behaviour, which means the C++ rules say nothing about what happens; the program may print random numbers or behave differently each run.

int total;          // garbage, do not read it
int count = 0;      // copy initialisation
int marks{85};      // brace initialisation
int bad{3.7};       // compile error: braces refuse to lose the .7

Integer division and overflow

std::cout << 7 / 2 << '\n';      // 3   (int / int drops the fraction)
std::cout << 7 / 2.0 << '\n';    // 3.5 (one double makes the result double)
std::cout << 7 % 2 << '\n';      // 1   (remainder)

int big = 2'000'000'000;          // ' is a digit separator since C++14
long long safe = 1LL * big * 2;   // 4000000000, computed in long long

Conversions (casts)

int total = 7, people = 2;
double each = static_cast<double>(total) / people;   // 3.5
int rounded = static_cast<int>(3.99);                  // 3, the fraction is cut off

const, constexpr and auto

• const int MAX_SEATS = 72; can never be changed after it is set. Trying to change it is a compile error.
• constexpr double GST = 0.18; is a constant the compiler knows at compile time, usable anywhere a fixed value is needed (like an array size).
• auto lets the compiler work out the type from the value: auto n = 5; is int, auto p = 2.5; is double. Use it when the type is obvious or very long.

Common mistakes

• Using = (assign) when you meant == (compare) in a condition.
• Expecting 5 / 2 to be 2.5. With two ints it is 2.
• Reading a variable before giving it a value.
• Storing big money totals or counts in int. Use long long for anything that may pass 2 billion.