Initializes an object from explicit set of constructor arguments.
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Tobject(arg);Tobject(arg1, arg2, ...); | (1) | |
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Tobject{arg}; | (2) | (since C++11) |
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T(other)T(arg1, arg2, ...) | (3) | |
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static_cast<T>(other) | (4) | |
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newT(args, ...) | (5) | |
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Class::Class():member(args, ...){...} | (6) | |
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[arg]() {...} | (7) | (since C++11) |
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[edit]Explanation
Direct-initialization is performed in the following situations:
1) Initialization with a nonempty parenthesized list of expressionsor braced-init-lists(since C++11).
2) 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, seelist-initialization)(since C++11).
3) 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.
4) Initialization of
a prvalue temporary(until C++17)the result object of a prvalue(since C++17) by a
static_cast expression.
5) Initialization of an object with dynamic storage duration by a new-expression with an initializer.
6) Initialization of a base or a non-static member by constructor
initializer list.
7) Initialization of closure object members from the variables caught by copy in a lambda-expression.
The effects of direct-initialization are:
- the program is ill-formed.
| (until C++20) |
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 the initializer is aprvalue 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: seecopy elision)
| (since C++17) |
- the constructors of
T are 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 inaggregate 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 arevalue-initialized.
struct B{int a;int&& r;}; int f();int n=10; B b1{1, f()};// OK, lifetime is extendedB b2(1, f());// well-formed, but dangling referenceB b3{1.0,1};// error: narrowing conversionB b4(1.0,1);// well-formed, but dangling referenceB b5(1.0, std::move(n));// OK | (since C++20) |
- Otherwise, if
T is 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
T isbool and the source type isstd::nullptr_t, the value of the initialized object isfalse. - Otherwise,standard conversions are used, if necessary, to convert the value ofother to the cv-unqualified version of
T, and the initial value of the object being initialized is the (possibly converted) value.
Direct-initialization is more permissive than copy-initialization: copy-initialization only considers non-explicit constructors and non-explicit user-definedconversion 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 afunction declaration, the compiler always chooses function declaration. This disambiguation rule is sometimes counter-intuitive and has been called themost vexing parse.
#include <fstream>#include <iterator>#include <string> int main(){std::ifstream file("data.txt"); // The following is a function declaration:std::string foo1(std::istreambuf_iterator<char>(file),std::istreambuf_iterator<char>());// It declares a function called foo1, whose return type is std::string,// first parameter has type std::istreambuf_iterator<char> and the name "file",// second parameter has no name and has type std::istreambuf_iterator<char>(),// which is rewritten to function pointer type std::istreambuf_iterator<char>(*)() // Pre-C++11 fix (to declare a variable) - add extra parentheses around one// of the arguments:std::string str1((std::istreambuf_iterator<char>(file)),std::istreambuf_iterator<char>()); // Post-C++11 fix (to declare a variable) - use list-initialization for any// of the arguments:std::string str2(std::istreambuf_iterator<char>{file},{});}[edit]Example
#include <iostream>#include <memory>#include <string> 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<int> p(newint(1));// OK: explicit constructors allowed// std::unique_ptr<int> 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:
[edit]See also
