Use the `T{e}`notation for construction

Reason

The T{e} construction syntax makes it explicit that construction is desired. The T{e} construction syntax doesn't allow narrowing. T{e} is the only safe and general expression for constructing a value of type T from an expression e. The casts notations T(e) and (T)e are neither safe nor general.

Example

For built-in types, the construction notation protects against narrowing and reinterpretation

void use(char ch, int i, double d, char* p, long long lng)
{
    int x1 = int{ch};     // OK, but redundant
    int x2 = int{d};      // error: double->int narrowing; use a cast if you need to
    int x3 = int{p};      // error: pointer to->int; use a reinterpret_cast if you really need to
    int x4 = int{lng};    // error: long long->int narrowing; use a cast if you need to

    int y1 = int(ch);     // OK, but redundant
    int y2 = int(d);      // bad: double->int narrowing; use a cast if you need to
    int y3 = int(p);      // bad: pointer to->int; use a reinterpret_cast if you really need to
    int y4 = int(lng);    // bad: long long->int narrowing; use a cast if you need to

    int z1 = (int)ch;     // OK, but redundant
    int z2 = (int)d;      // bad: double->int narrowing; use a cast if you need to
    int z3 = (int)p;      // bad: pointer to->int; use a reinterpret_cast if you really need to
    int z4 = (int)lng;    // bad: long long->int narrowing; use a cast if you need to
}

The integer to/from pointer conversions are implementation defined when using the T(e) or (T)e notations, and non-portable between platforms with different integer and pointer sizes.

Note

Avoid casts (explicit type conversion) and if you must prefer named casts.

Note

When unambiguous, the T can be left out of T{e}.

complex<double> f(complex<double>);

auto z = f({2*pi, 1});

Note

The construction notation is the most general initializer notation.

Exception

std::vector and other containers were defined before we had {} as a notation for construction. Consider:

vector<string> vs {10};                           // ten empty strings
vector<int> vi1 {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};  // ten elements 1..10
vector<int> vi2 {10};                             // one element with the value 10

How do we get a vector of 10 default initialized ints?

vector<int> v3(10); // ten elements with value 0

The use of () rather than {} for number of elements is conventional (going back to the early 1980s), hard to change, but still a design error: for a container where the element type can be confused with the number of elements, we have an ambiguity that must be resolved. The conventional resolution is to interpret {10} as a list of one element and use (10) to distinguish a size.

This mistake need not be repeated in new code. We can define a type to represent the number of elements:

struct Count { int n; };

template<typename T>
class Vector {
public:
    Vector(Count n);                     // n default-initialized elements
    Vector(initializer_list<T> init);    // init.size() elements
    // ...
};

Vector<int> v1{10};
Vector<int> v2{Count{10}};
Vector<Count> v3{Count{10}};    // yes, there is still a very minor problem

The main problem left is to find a suitable name for Count.

Enforcement

Flag the C-style (T)e and functional-style T(e) casts.