Direct-initialization - cppreference.com (original) (raw)
Initializes an object from explicit set of constructor arguments.
[edit] Syntax
| T object ( arg ); T object ( arg1, arg2, ... ); | (1) | |
| T object { arg }; | (2) | (since C++11) |
| T ( other ) T ( arg1, arg2, ... ) | (3) | |
| static_cast<** T **>( other ) | (4) | |
| new T**(** args, ... ) | (5) | |
| Class**::Class()** : member**(** args, ... ) { ... } | (6) | |
| [**arg**]() { ... } | (7) | (since C++11) |
[edit] Explanation
Direct-initialization is performed in the following situations:
Initialization with a nonempty parenthesized list of expressions or braced-init-lists(since C++11).
Initialization of an object of non-class type with a single brace-enclosed initializer (note: for class types and other uses of braced-init-list, see list-initialization)(since C++11).
Initialization of a prvalue temporary(until C++17)the result object of a prvalue(since C++17) by function-style cast or with a parenthesized expression list.
Initialization of a prvalue temporary(until C++17)the result object of a prvalue(since C++17) by a static_cast expression.
Initialization of an object with dynamic storage duration by a new-expression with an initializer.
Initialization of a base or a non-static member by constructor initializer list.
Initialization of closure object members from the variables caught by copy in a lambda-expression.
The effects of direct-initialization are:
- If
Tis an array type,
| the program is ill-formed. | (until C++20) |
|---|---|
| the array is initialized as in aggregate initialization, except that narrowing conversions are allowed and any elements without an initializer are value-initialized. struct A { explicit A(int i = 0) {} }; A a[2](A(1)); // OK: initializes a[0] with A(1) and a[1] with A() A b[2]{A(1)}; // error: implicit copy-list-initialization of b[1] // from {} selected explicit constructor | (since C++20) |
- If
Tis a class type,
| if the initializer is a prvalue expression whose type is the same class as T (ignoring cv-qualification), the initializer expression itself, rather than a temporary materialized from it, is used to initialize the destination object.(Before C++17, the compiler may elide the construction from the prvalue temporary in this case, but the appropriate constructor must still be accessible: see copy elision) | (since C++17) |
|---|
- the constructors of
Tare examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.
| otherwise, if the destination type is a (possibly cv-qualified) aggregate class, it is initialized as described in aggregate initialization except that narrowing conversions are permitted, designated initializers are not allowed, a temporary bound to a reference does not have its lifetime extended, there is no brace elision, and any elements without an initializer are value-initialized. struct B { int a; int&& r; }; int f(); int n = 10; B b1{1, f()}; // OK, lifetime is extended B b2(1, f()); // well-formed, but dangling reference B b3{1.0, 1}; // error: narrowing conversion B b4(1.0, 1); // well-formed, but dangling reference B b5(1.0, std::move(n)); // OK | (since C++20) |
|---|
- Otherwise, if
Tis a non-class type but the source type is a class type, the conversion functions of the source type and its base classes, if any, are examined and the best match is selected by overload resolution. The selected user-defined conversion is then used to convert the initializer expression into the object being initialized. - Otherwise, if
Tis bool and the source type is std::nullptr_t, the value of the initialized object is false. - Otherwise, standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version of
T, and the initial value of the object being initialized is the (possibly converted) value.
[edit] Notes
Direct-initialization is more permissive than copy-initialization: copy-initialization only considers non-explicit constructors and non-explicit user-defined conversion functions, while direct-initialization considers all constructors and all user-defined conversion functions.
In case of ambiguity between a variable declaration using the direct-initialization syntax (1) (with round parentheses) and a function declaration, the compiler always chooses function declaration. This disambiguation rule is sometimes counter-intuitive and has been called the most vexing parse.
#include #include #include int main() { std::ifstream file("data.txt"); // The following is a function declaration: std::string foo1(std::istreambuf_iterator(file), std::istreambuf_iterator()); // It declares a function called foo1, whose return type is std::string, // first parameter has type std::istreambuf_iterator and the name "file", // second parameter has no name and has type std::istreambuf_iterator(), // which is rewritten to function pointer type std::istreambuf_iterator(*)() // Pre-C++11 fix (to declare a variable) - add extra parentheses around one // of the arguments: std::string str1((std::istreambuf_iterator(file)), std::istreambuf_iterator()); // Post-C++11 fix (to declare a variable) - use list-initialization for any // of the arguments: std::string str2(std::istreambuf_iterator{file}, {}); }
[edit] Example
#include #include #include struct Foo { int mem; explicit Foo(int n) : mem(n) {} }; int main() { std::string s1("test"); // constructor from const char* std::string s2(10, 'a'); std::unique_ptr p(new int(1)); // OK: explicit constructors allowed // std::unique_ptr p = new int(1); // error: constructor is explicit Foo f(2); // f is direct-initialized: // constructor parameter n is copy-initialized from the rvalue 2 // f.mem is direct-initialized from the parameter n // Foo f2 = 2; // error: constructor is explicit std::cout << s1 << ' ' << s2 << ' ' << *p << ' ' << f.mem << '\n'; }
Output: