C++: Variables & Data Types

What you will learn

How C++ declares variables with explicit types, the difference between primitive types and std::string, and how type sizes vary by platform.

C++ is statically typed — the compiler checks types at compile time. Every variable must have a declared type, and that type cannot change.

#include <string>

int age = 25;                      // int
double height = 5.6;               // double (floating point)
bool isStudent = true;             // bool
std::string name = "Alice";       // std::string from <string> header
char initial = 'A';                // char (single character)

Common types and their sizes

Type	Typical Size	Example	Notes
`int`	4 bytes	`42`	Platform-dependent (usually 32-bit)
`double`	8 bytes	`3.14`	Double-precision float
`float`	4 bytes	`2.5f`	Single-precision (use `f` suffix)
`bool`	1 byte	`true`	Can be `true` or `false`
`char`	1 byte	`'A'`	ASCII character (use single quotes)
`std::string`	variable	`"hello"`	Object from `<string>` header (use double quotes)

std::string is not a primitive — it is a class from the C++ Standard Library. You must #include <string> to use it.

auto keyword (C++11+)

C++11 introduced type inference with auto:

auto x = 42;          // int
auto y = 3.14;        // double
auto name = "Alice";  // const char* (not std::string!)

Watch out: auto for a string literal gives const char*, not std::string. Use auto s = std::string{"hello"}; if you want the string type.

Common mistakes

Forgetting #include <string> and getting a compile error about std::string being unknown
Using single quotes for strings ('hello') — single quotes are for char, double quotes for string literals
Assuming int is always 32 bits — on some embedded platforms int can be 16 bits; use <cstdint> types like int32_t for portability
Confusing = (assignment) with == (equality) in conditions

Quick check below!