Prefer the `{}`-initializer syntax

Reason

Prefer {}. The rules for {} initialization are simpler, more general, less ambiguous, and safer than for other forms of initialization.

Use = only when you are sure that there can be no narrowing conversions. For built-in arithmetic types, use = only with auto.

Avoid () initialization, which allows parsing ambiguities.

Example

int x {f(99)};
int y = x;
vector<int> v = {1, 2, 3, 4, 5, 6};

Exception

For containers, there is a tradition for using {...} for a list of elements and (...) for sizes:

vector<int> v1(10);    // vector of 10 elements with the default value 0
vector<int> v2{10};    // vector of 1 element with the value 10

vector<int> v3(1, 2);  // vector of 1 element with the value 2
vector<int> v4{1, 2};  // vector of 2 elements with the values 1 and 2

Note

{}-initializers do not allow narrowing conversions (and that is usually a good thing) and allow explicit constructors (which is fine, we're intentionally initializing a new variable).

Example

int x {7.9};   // error: narrowing
int y = 7.9;   // OK: y becomes 7. Hope for a compiler warning
int z {gsl::narrow_cast<int>(7.9)};    // OK: you asked for it
auto zz = gsl::narrow_cast<int>(7.9);  // OK: you asked for it

Note

{} initialization can be used for nearly all initialization; other forms of initialization can't:

auto p = new vector<int> {1, 2, 3, 4, 5};   // initialized vector
D::D(int a, int b) :m{a, b} {   // member initializer (e.g., m might be a pair)
    // ...
};
X var {};   // initialize var to be empty
struct S {
    int m {7};   // default initializer for a member
    // ...
};

For that reason, {}-initialization is often called "uniform initialization" (though there unfortunately are a few irregularities left).

Note

Initialization of a variable declared using auto with a single value, e.g., {v}, had surprising results until C++17. The C++17 rules are somewhat less surprising:

auto x1 {7};        // x1 is an int with the value 7
auto x2 = {7};      // x2 is an initializer_list<int> with an element 7

auto x11 {7, 8};    // error: two initializers
auto x22 = {7, 8};  // x22 is an initializer_list<int> with elements 7 and 8

Use ={...} if you really want an initializer_list<T>

auto fib10 = {1, 1, 2, 3, 5, 8, 13, 21, 34, 55};   // fib10 is a list

Note

={} gives copy initialization whereas {} gives direct initialization. Like the distinction between copy-initialization and direct-initialization itself, this can lead to surprises. {} accepts explicit constructors; ={} does not. For example:

struct Z { explicit Z() {} };

Z z1{};     // OK: direct initialization, so we use explicit constructor
Z z2 = {};  // error: copy initialization, so we cannot use the explicit constructor

Use plain {}-initialization unless you specifically want to disable explicit constructors.

Example

template<typename T>
void f()
{
    T x1(1);    // T initialized with 1
    T x0();     // bad: function declaration (often a mistake)

    T y1 {1};   // T initialized with 1
    T y0 {};    // default initialized T
    // ...
}

See also: Discussion

Enforcement

• Flag uses of = to initialize arithmetic types where narrowing occurs.
• Flag uses of () initialization syntax that are actually declarations. (Many compilers should warn on this already.)