Converts between types using a combination of explicit and implicit conversions.
Syntax
(type-id)unary-expression(1) simple-type-specifier(expression-list(optional))
simple-type-specifier(initializer-list(optional))(2) (until C++11)
(since C++11)simple-type-specifier{initializer-list(optional)}(3) (since C++11)simple-type-specifier{designated-initializer-list}(4) (since C++20)typenameidentifier(initializer-list(optional))(5) (since C++11)typenameidentifier{initializer-list(optional)}(6) (since C++11)typenameidentifier{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.
type-id- a
unary-expression- an unary expression (whose top-level operator does not have a
higher than that of C-style cast) simple-type-specifier- a
expression-list- a comma-separated list of expressions (except unparenthesized
) initializer-list- a comma-separated list of
designated-initializer-list- a comma-separated list of
designated initializer clauses
identifier- a (possibly qualified) identifier (including
) Explanation
1) When the C-style cast is encountered, the compiler attempts to interpret it as the following cast expressions, in this order:
a)
<type-id>(unary-expression);
b)
<type-id>(unary-expression), with extensions: pointer or reference to a
is additionally allowed to be cast to pointer or reference to unambiguous base class (and vice versa) even if the base class is
(that is, this cast ignores the private inheritance specifier). Same applies to casting
to 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.
(until C++17)T is determined as follows:
If the specified type is a placeholder for a deduced class type, T is the return type of the function selected by overload resolution for
.
Otherwise, if the specified type contains a
, T is the deduced type.
(since C++23)Otherwise, T is the specified type.
(since C++17) The conversion result is determined as follows:
If the function-style cast is of syntax (2), and there is exactly one expression in parentheses, this cast is equivalent to the corresponding C-style cast.
Otherwise, if 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.
If the initializer is not (), the program is ill-formed.
(until C++11)If the initializer is not () or {} after
(if any), the program is ill-formed.
(since C++11)Otherwise, if T is a reference type, the function-style cast has the same effect as
an invented variable t of type T from the specified initializer, and the result is the initialized t.
The result is an lvalue.
(until C++11)If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue.
Otherwise, the result is an xvalue.
(since C++11)Otherwise, the result is an rvalue(until C++11)a prvalue(since C++11) of of type Tdesignating a temporary(until C++17)whose result object is(since C++17)
with the specified initializer.
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:
structM{};structL{L(M&);};Mn;voidf(){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
, the statement will only be treated as a declaration statement if the trailing return type starts with auto:
structM;structS{S*operator()();intN;intM;voidmem(Ss){auto(s)()->M;// expression (S::M hides ::M), invalid before C++23}};voidf(Ss){{auto(s)()->N;// expression, invalid before C++23auto(s)()->M;// function declaration, equivalent to M s();}{S(s)()->N;// expressionS(s)()->M;// expression}}(since C++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:
structS{S(int);};voidfoo(doublea){Sw(int(a));// function declaration: has a parameter `a` of type intSx(int());// function declaration: has an unnamed parameter of type int(*)() // that is adjusted from int()// Ways to avoid ambiguity:Sy((int(a)));// object declaration: extra pair of parenthesesSy((int)a);// object declaration: C-style castSz=int(a);// object declaration: no ambiguity for this syntax}However, if the outermost declarator in the ambiguous parameter declaration has a
, the ambiguity will only be resolved by treating it as a declaration if it starts with auto:
typedefstructBB{intC[2];}*B,C;voidfoo(){Sa(B()->C);// object declaration: B()->C cannot declare a parameterSb(auto()->C);// function declaration: has an unnamed parameter of type C(*)()// that is adjusted from C()}(since C++11)Ambiguous type-id
An ambiguity can arise from the similarity between a function-style cast and a
. 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 unsignedvoidfoo(signedchara){sizeof(int());// type-id (ill-formed)sizeof(int(a));// expressionsizeof(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
has a
, the ambiguity will only be resolved by treating it as a type-id if it starts with auto:
typedefstructBB{intC[2];}*B,C;voidfoo(){sizeof(B()->C[1]);// OK, sizeof(expression)sizeof(auto()->C[1]);// error: sizeof of a function returning an array}(since C++11)Notes
Feature-test macroValueStdFeature
(C++23)auto(x) and auto{x}Example
Run this code
#include<cassert>#include<iostream>doublef=3.14;unsignedintn1=(unsignedint)f;// C-style castunsignedintn2=unsigned(f);// function-style castclassC1;classC2;C2*foo(C1*p){return(C2*)p;// casts incomplete type to incomplete type}voidcpp23_decay_copy_demo(){autoinc_print=[](int&x,constint&y){++x;std::cout<<"x:"<<x<<", y:"<<y<<'\n';};intp{1};inc_print(p,p);// prints x:2 y:2, because param y here is an alias of pintq{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_caststructA{};structI1:A{};structI2:A{};structD:I1,I2{};intmain(){D*d=nullptr;// A* a = (A*)d; // compile-time errorA*a=reinterpret_cast<A*>(d);// this compilesassert(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 C++ standards.
DR Applied to Behavior as published Correct behavior
(
) C++11 the addition of trailing return type introduced more ambiguities resolves them
C++11 function-style cast did not consider pack expansions considers them
C++11 void{} was ill-formed made well-formed
C++98 the resolution of ambiguous function
parameters might be misinterpreted improved the wording
C++98 a C-style cast was ill-formed if multiple interpretations
of a static_cast followed by a const_cast exist,
regardless of whether these conversions are actually used only considers the
conversions
possibly being used
C++98 function-style casts could create reference rvalues can only create reference lvalues References
C++23 standard (ISO/IEC 14882:2024):
7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]
7.6.3 Explicit type conversion (cast notation) [expr.cast]
C++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]
C++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]
C++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]
C++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]
C++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]
C++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
adds or removes const
performs basic conversions
performs checked polymorphic conversions
performs general low-level conversions
implicit conversions from one type to another
for cast operator