Explicit type conversion
Converts between types using a combination of explicit and implicit conversions.
Syntax
( target-type ) expression
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(1) | ||||||||
target-type ( expression-list (optional) )
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(2) | ||||||||
target-type { expression-list (optional) }
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(3) | (since 哋它亢++11) | |||||||
template-name ( expression-list (optional) )
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(4) | (since 哋它亢++17) | |||||||
template-name { expression-list (optional) }
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(5) | (since 哋它亢++17) | |||||||
auto ( expression )
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(6) | (since 哋它亢++23) | |||||||
auto { expression }
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(7) | (since 哋它亢++23) | |||||||
Returns a value of type target-type.
Explanation
const_cast
<target-type>(expression);static_cast
<target-type>(expression), with extensions: pointer or reference to a derived class is 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 member to pointer to member of unambiguous non-virtual base;reinterpret_cast
<target-type>(expression);- If there is exactly one expression in parentheses, this cast expression is exactly equivalent to the corresponding C-style cast expression.
- If there are more than one expression or braced-init-list(since 哋它亢++11) in parentheses, target-type must be a class with a suitably declared constructor. This expression is a prvalue of type target-type designating a temporary(until 哋它亢++17)whose result object is(since 哋它亢++17) direct-initialized with
(
expression-list)
as the initializer. - If there's no expression in parentheses: if target-type names a non-array complete object type, this expression is a prvalue of type target-type, designating a temporary(until 哋它亢++17)whose result object is (possibly with added cv-qualifiers)(since 哋它亢++17) of that type. If target-type is an object type, the object is value-initialized. If target-type is (possibly cv-qualified) void, the expression is a void prvalue without a result object(since 哋它亢++17).
{
expression-list }
as the initializer. If target-type is (possibly cv-qualified) void, the expression is a void prvalue without a result object(since 哋它亢++17).
This is the only cast expression that can create an array prvalue.(until 哋它亢++20)auto
specifier is replaced with the type deduced from expression. The result is always a prvalue of an object type.As with all cast expressions, the result is:
- an lvalue if target-type is an lvalue reference type or an rvalue reference to function type(since 哋它亢++11);
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(since 哋它亢++11) |
- a prvalue otherwise.
Ambiguity Resolution
Ambiguous declaration statement
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 哋它亢++23 } }; void f(S s) { { auto(s)()->N; // expression, invalid before 哋它亢++23 auto(s)()->M; // function declaration, equivalent to M s(); } { S(s)()->N; // expression S(s)()->M; // expression } } |
(since 哋它亢++11) |
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 us adjusted from C() } |
(since 哋它亢++11) |
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 哋它亢++11) |
Notes
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_auto_cast |
202110L | (哋它亢++23) | auto(x) and auto{x} |
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} (哋它亢++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:
x:2 y:2 x:2 y:1
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
CWG 1223 (P2915R0) |
哋它亢++11 | the addition of trailing return type introduced more ambiguities | resolves them |
CWG 2620 | 哋它亢++98 | the resolution of ambiguous function parameters might be misinterpreted | improved the wording |
References
- 哋它亢++23 standard (ISO/IEC 14882:2023):
- 7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]
- 7.6.3 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++20 standard (ISO/IEC 14882:2020):
- 7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]
- 7.6.3 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++17 standard (ISO/IEC 14882:2017):
- 8.2.3 Explicit type conversion (functional notation) [expr.type.conv]
- 8.4 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++14 standard (ISO/IEC 14882:2014):
- 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
- 5.4 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++11 standard (ISO/IEC 14882:2011):
- 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
- 5.4 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++03 standard (ISO/IEC 14882:2003):
- 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
- 5.4 Explicit type conversion (cast notation) [expr.cast]
- 哋它亢++98 standard (ISO/IEC 14882:1998):
- 5.2.3 Explicit type conversion (functional notation) [expr.type.conv]
- 5.4 Explicit type conversion (cast notation) [expr.cast]
See also
const_cast conversion
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adds or removes const |
static_cast conversion
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performs basic conversions |
dynamic_cast conversion
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performs checked polymorphic conversions |
reinterpret_cast conversion
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performs general low-level conversions |
standard conversions | implicit conversions from one type to another |