explicit specifier

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	catch handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Classes

General

Overview

class/struct types

union types

Injected-class-name

Class property specifiers (C++26)

Members

Data members

Static members

The this pointer

Nested classes

Member templates

Bit-fields

using-declarations

Member functions

Member access specifiers

Constructors and member initializer lists

Default member initializer (C++11)

friend specifier

explicit specifier

Converting constructor

Special member functions

Default constructor

Copy constructor

Move constructor (C++11)

Copy assignment operator

Move assignment operator (C++11)

Destructor

Inheritance

Base and derived classes

Empty base optimization (EBO)

Virtual member functions

Pure virtual functions and abstract classes

override specifier (C++11)

final specifier (C++11)

 

Syntax

explicit	(1)
explicit ( expression )	(2)	(since C++20)

expression

-

contextually converted constant expression of type bool

1) Specifies that a constructor or conversion function(since C++11)or deduction guide(since C++17) is explicit, that is, it cannot be used for implicit conversions and copy-initialization.

2) The explicit specifier may be used with a constant expression. The function is explicit if and only if that constant expression evaluates to true.

(since C++20)

The explicit specifier may only appear within the decl-specifier-seq of the declaration of a constructor or conversion function(since C++11) within its class definition.

Notes

A constructor with a single non-default parameter(until C++11) that is declared without the function specifier explicit is called a converting constructor.

Both constructors (other than copy/move) and user-defined conversion functions may be function templates; the meaning of explicit does not change.

A ( token that follows explicit is always parsed as part of the explicit specifier: struct S { explicit (S)(const S&); / error in C++20, OK in C++17 explicit (operator int)(); / error in C++20, OK in C++17 };

(since C++20)

Feature-test macro	Value	Std	Feature
__cpp_conditional_explicit	201806L	(C++20)	conditional explicit

Keywords

explicit

Example

Run this code

struct A
{
    A(int) {}      / converting constructor
    A(int, int) {} / converting constructor (C++11)
    operator bool() const { return true; }
};
 
struct B
{
    explicit B(int) {}
    explicit B(int, int) {}
    explicit operator bool() const { return true; }
};
 
int main()
{
    A a1 = 1;      / OK: copy-initialization selects A::A(int)
    A a2(2);       / OK: direct-initialization selects A::A(int)
    A a3 {4, 5};   / OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4, 5}; / OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1;   / OK: explicit cast performs static_cast
    if (a1) { }    / OK: A::operator bool()
    bool na1 = a1; / OK: copy-initialization selects A::operator bool()
    bool na2 = static_cast<bool>(a1); / OK: static_cast performs direct-initialization
 
/  B b1 = 1;      / error: copy-initialization does not consider B::B(int)
    B b2(2);       / OK: direct-initialization selects B::B(int)
    B b3 {4, 5};   / OK: direct-list-initialization selects B::B(int, int)
/  B b4 = {4, 5}; / error: copy-list-initialization does not consider B::B(int, int)
    B b5 = (B)1;   / OK: explicit cast performs static_cast
    if (b2) { }    / OK: B::operator bool()
/  bool nb1 = b2; / error: copy-initialization does not consider B::operator bool()
    bool nb2 = static_cast<bool>(b2); / OK: static_cast performs direct-initialization
 
    [](...){}(a4, a5, na1, na2, b5, nb2); / suppresses “unused variable” warnings
}

See also

• converting constructor
• initialization
• copy initialization
• direct initialization

Follow Lee on X/Twitter - Father, Husband, Serial builder creating AI, crypto, games & web tools. We are friends :) AI Will Come To Life!

Check out: eBank.nz (Art Generator) | Netwrck.com (AI Tools) | Text-Generator.io (AI API) | BitBank.nz (Crypto AI) | ReadingTime (Kids Reading) | RewordGame | BigMultiplayerChess | WebFiddle | How.nz | Helix AI Assistant