Converts between types using a combination of explicit and implicit conversions.
[edit] Syntax( type-id ) unary-expression (1) simple-type-specifier ( expression-list (optional) )
( initializer-list (optional) ) (2) (until C++11)
{ initializer-list (optional) } (3) (since C++11) simple-type-specifier { designated-initializer-list } (4) (since C++20) typename identifier ( initializer-list (optional) ) (5) (since C++11) typename identifier { initializer-list (optional) } (6) (since C++11) typename identifier { designated-initializer-list } (7) (since C++20)
Explicitly converts any number of values to a value of the target type.
1) Explicit type conversion (cast notation), also called C-style cast.
2-7) Explicit type conversion (functional notation), also called function-style cast.
[edit] Explanation1) When the C-style cast is encountered, the compiler attempts to interpret it as the following cast expressions, in this order:
b)static_cast<type-id >(unary-expression )
, with extensions: pointer or reference to a
derived classis additionally allowed to be cast to pointer or reference to unambiguous base class (and vice versa) even if the base class is
inaccessible(that is, this cast ignores the private inheritance specifier). Same applies to casting
pointer to memberto pointer to member of unambiguous non-virtual base;
c) a static_cast (with extensions) followed by const_cast;
e) a reinterpret_cast followed by const_cast.
The first choice that satisfies the requirements of the respective cast operator is selected, even if it is ill-formed (see example). If a static_cast followed by a const_cast is used and the conversion can be interpreted in more than one way as such, the conversion is ill-formed.
In addition, C-style casts can cast from, to, and between pointers to incomplete class type. If both type-id and the type of unary-expression are pointers to incomplete class types, it is unspecified whether static_cast or reinterpret_cast gets selected.
2-7)A function-style cast specifies a
type(
simple-type-specifier
or identifier (since C++11)) and an
initializer(the remaining parts), it constructs a value of the target type
T
, which is determined from the specified type
and initializer(since C++17):
T is the specified type.
T is determined as follows:
T is the return type of the function selected by overload resolution for class template deduction.T is the deduced type.T is the specified type.The conversion result is determined as follows:
T is (possibly cv-qualified) void, the result is an rvalue(until C++11)a prvalue(since C++11) of type void that performs no initialization.T is a reference type, the function-style cast has the same effect as direct-initializing an invented variable t of type T from the specified initializer, and the result is the initialized t.T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue.T designating a temporary(until C++17)whose result object is(since C++17) direct-initialized with the specified initializer.In the case of an ambiguity between an expression statement with a function-style cast expression as its leftmost subexpression and a declaration statement, the ambiguity is resolved by treating it as a declaration. This disambiguation is purely syntactic: it does not consider the meaning of names occurring in the statement other than whether they are type names:
struct M {};
struct L { L(M&); };
M n;
void f()
{
M(m); // declaration, equivalent to M m;
L(n); // ill-formed declaration, equivalent to L n;
L(l)(m); // still a declaration, equivalent to L l((m));
}
However, if the outermost declarator in the ambiguous declaration statement has a trailing return type, the statement will only be treated as a declaration statement if the trailing return type starts with auto:
struct M;
struct S
{
S* operator()();
int N;
int M;
void mem(S s)
{
auto(s)()->M; // expression (S::M hides ::M), invalid before C++23
}
};
void f(S s)
{
{
auto(s)()->N; // expression, invalid before C++23
auto(s)()->M; // function declaration, equivalent to M s();
}
{
S(s)()->N; // expression
S(s)()->M; // expression
}
}(since C++11) [edit] Ambiguous function parameter
The ambiguity above can also occur in the context of a declaration. In that context, the choice is between an object declaration with a function-style cast as the initializer and a declaration involving a function declarator with a redundant set of parentheses around a parameter name. The resolution is also to consider any construct, such as the potential parameter declaration, that could possibly be a declaration to be a declaration:
struct S
{
S(int);
};
void foo(double a)
{
S w(int(a)); // function declaration: has a parameter `a` of type int
S x(int()); // function declaration: has an unnamed parameter of type int(*)()
// that is adjusted from int()
// Ways to avoid ambiguity:
S y((int(a))); // object declaration: extra pair of parentheses
S y((int)a); // object declaration: C-style cast
S z = int(a); // object declaration: no ambiguity for this syntax
}
However, if the outermost declarator in the ambiguous parameter declaration has a trailing return type, the ambiguity will only be resolved by treating it as a declaration if it starts with auto:
typedef struct BB { int C[2]; } *B, C;
void foo()
{
S a(B()->C); // object declaration: B()->C cannot declare a parameter
S b(auto()->C); // function declaration: has an unnamed parameter of type C(*)()
// that is adjusted from C()
}(since C++11) [edit] Ambiguous type-id
An ambiguity can arise from the similarity between a function-style cast and a type-id. The resolution is that any construct that could possibly be a type-id in its syntactic context shall be considered a type-id:
// `int()` and `int(unsigned(a))` can both be parsed as type-id:
// `int()` represents a function returning int
// and taking no argument
// `int(unsigned(a))` represents a function returning int
// and taking an argument of type unsigned
void foo(signed char a)
{
sizeof(int()); // type-id (ill-formed)
sizeof(int(a)); // expression
sizeof(int(unsigned(a))); // type-id (ill-formed)
(int()) + 1; // type-id (ill-formed)
(int(a)) + 1; // expression
(int(unsigned(a))) + 1; // type-id (ill-formed)
}
However, if the outermost abstract-declarator in the ambiguous type-id has a trailing return type, the ambiguity will only be resolved by treating it as a type-id if it starts with auto:
typedef struct BB { int C[2]; } *B, C;
void foo()
{
sizeof(B()->C[1]); // OK, sizeof(expression)
sizeof(auto()->C[1]); // error: sizeof of a function returning an array
}(since C++11) [edit] Notes [edit] Example
#include <cassert>
#include <iostream>
double f = 3.14;
unsigned int n1 = (unsigned int)f; // C-style cast
unsigned int n2 = unsigned(f); // function-style cast
class C1;
class C2;
C2* foo(C1* p)
{
return (C2*)p; // casts incomplete type to incomplete type
}
void cpp23_decay_copy_demo()
{
auto inc_print = [](int& x, const int& y)
{
++x;
std::cout << "x:" << x << ", y:" << y << '\n';
};
int p{1};
inc_print(p, p); // prints x:2 y:2, because param y here is an alias of p
int q{1};
inc_print(q, auto{q}); // prints x:2 y:1, auto{q} (C++23) casts to prvalue,
// so the param y is a copy of q (not an alias of q)
}
// In this example, C-style cast is interpreted as static_cast
// even though it would work as reinterpret_cast
struct A {};
struct I1 : A {};
struct I2 : A {};
struct D : I1, I2 {};
int main()
{
D* d = nullptr;
// A* a = (A*)d; // compile-time error
A* a = reinterpret_cast<A*>(d); // this compiles
assert(a == nullptr);
cpp23_decay_copy_demo();
}
Output:
[edit] Defect reportsThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior CWG 1223RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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