Variable initialization | C++ Quick Reference

At a glance

Syntax	Name	Result
`int x;`	Default initialization	No initial value is provided. For a local `int`, the value is indeterminate.
`int x = 5;`	Copy initialization	Initializes `x` with `5`. Allows narrowing conversions.
`int x(5);`	Direct initialization	Initializes `x` with `5`. Allows narrowing conversions.
`int x { 5 };`	Direct-list initialization	Initializes `x` with `5` and rejects narrowing conversions.
`int x {};`	Value initialization	Initializes `x` to zero.

int count { 5 };
int total {}; // 0

int x;

For a local fundamental type, no usable initial value is provided. Reading x before giving it a value results in undefined behavior.

Brace initialization rejects a value that cannot be represented safely by the destination type.

int a = 4.5;  // allowed: a becomes 4
int b(4.5);   // allowed: b becomes 4
int c { 4.5 }; // error: narrowing conversion

int x();

This declares a function named x that takes no arguments and returns an int. It does not define an integer variable.