C++ Variables

A variable is a named place in memory with a type. The type says which values it can keep and how much memory it uses.

Declare and initialize

#include <iostream>
#include <string>

int main() {
    int age = 21;                 // copy initialization
    double price{99.5};           // brace initialization (preferred)
    std::string name = "Riya";
    bool active = true;
    char grade = 'A';

    std::cout << name << " " << age << " " << price << " " << active << " " << grade << '\n';
}

Always initialize

A local variable without a value keeps garbage from the memory. Reading it is undefined behavior. The compiler does not stop you, but with -Wall it warns you (this is the real warning text):

#include <iostream>

int main() {
    int x;
    std::cout << x << '\n';
}

Brace initialization stops narrowing

int main() {
    int a = 3.7;       // allowed, the value becomes 3 (maybe a warning)
    int b{3.7};        // error: narrowing conversion
}

auto: let the compiler find the type

#include <iostream>
#include <typeinfo>

int main() {
    auto count = 10;          // int
    auto rate = 4.5;          // double
    auto text = "hi";         // const char*
    std::cout << typeid(count).name() << " " << typeid(rate).name() << " " << typeid(text).name() << '\n';
}

The letters are the compiler code of the types (i = int, d = double, PKc = pointer to const char). auto is still strongly typed.

const and constexpr

int main() {
    const double PI = 3.14159;
    PI = 3;
}

const means the value can not change after the start. constexpr means the value is known when the program is compiled, so it can be used for array sizes and in other compile time work.

#include <array>
#include <iostream>

constexpr int size = 4;

int main() {
    std::array<int, size> a{1, 2, 3, 4};
    std::cout << a.size() << '\n';
}

Scope

A variable lives only inside the block { } where you declare it. A variable declared inside the for line lives only inside the loop.

Naming rules

• Letters, digits and _. Not starting with a digit.
• Do not start with an underscore and a capital letter, or use two underscores: these names are reserved.
• Use meaningful names: total_price, not tp.

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