Notes

A variable is a name attached to a chunk of memory of a known size. The type decides that size and how the bits get interpreted.

Code

int a = 5;        // 4 bytes, signed integer
float b = 3.14f;  // 4 bytes, floating point
double c = 3.14;  // 8 bytes, more precision
char d = 'x';     // 1 byte
bool e = true;    // 1 byte (usually)

Same number of bits, different meaning depending on type — the bit pattern for int 5 and float 5.0f look nothing alike.

Initialization matters

int x;        // uninitialized - garbage, whatever was in memory before
int y = 0;    // explicitly zero
int z{};      // brace-init, also zero, and warns on narrowing conversions

An uninitialized local variable is not zero. It’s whatever garbage happened to be sitting at that address. Always initialize.

Related

Const keyword
Operators and expressions