Scope

 

C++

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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++

 

Basic Concepts

Comments

ASCII

Punctuation

Names and identifiers

Types

Fundamental types

Objects

Scope

Object lifetime

Storage duration and linkage

Definitions and ODR

Name lookup

Qualified name lookup

Unqualified name lookup

The as-if rule

Undefined behavior

Memory model

Multi-threaded executions and data races(C++11)

Character sets and encodings

Phases of translation

The main function

Modules(C++20)

Contracts(C++26)

 

Each declaration that appears in a C++ program is only visible in some possibly discontiguous scopes .

Within a scope, unqualified name lookup can be used to associate a name with its declaration.

General

Each program has a global scope , which contains the entire program.

Every other scope S is introduced by one of the following:

• a declaration
• a parameter in parameter list
• a statement
• a handler

a contract assertion

(since C++26)

S always appear in another scope, which thereby containsS.

An enclosing scope at a program point is any scope that contains it; the smallest such scope is said to be the immediate scope at that point.

A scope intervenes between a program point P and a scope S (that does not contain P) if it is or contains S but does not contain P.

The parent scope of any scope S that is not a template parameter scope is the smallest scope that contains S and is not a template parameter scope.

Unless otherwise specified:

• A declaration inhabits the immediate scope at its locus.
• A declaration’s target scope is the scope it inhabits.
• Any names (re)introduced by a declaration are bound to it in its target scope.

An entity belongs to a scope S if S is the target scope of a declaration of the entity.

// global scope scope// scope S Tint x;// ─┐ // program point X// │{// │ ─┐{// │ │ ─┐int y;// │ │ │ // program point Y}// │ │ ─┘}// ─┘ ─┘

In the program above:

• The global scope, scope S and scope T contains program point Y.

• In other words, these three scopes are all enclosing scopes at program point Y.

• The global scope contains scopes S and T, and scope S contains scope T.

• Therefore, scope T is the smallest scope among all three, which means:

• Scope T is the immediate scope at program point Y.
• The declaration of the variable y inhabits scope T at its locus.
• Scope T is the target scope of the declaration of y.
• The variable y belongs to scope T.

• Scope S is the parent scope of scope T, and the global scope is the parent scope of scope S.

• Scope S intervenes between program point X and scope T.

Block scope

Each

• selection statement (if, switch),
• iteration statement (for, range-for(since C++11), while, do-while),
• handler, or
• compound statement that is not the compound-statement of a handler

introduces a block scope that includes the statement or handler.

A variable that belongs to a block scope is a block variable .

int i =42;int a[10];   for(int i =0; i <10; i++)// inner “i” inhabits the block scope a[i]= i;// introduced by the for-statement   int j = i;// j = 42

If the declaration inhabits a block scope S and declares a function or uses the extern specifier, the declaration shall not be attached to a named module (since C++20), its target scope is a larger enclosing scope (the innermost enclosing namespace scope), but the name is bound in their immediate scope S.

If a declaration that is not a name-independent declaration and(since C++26) that binds a name in the block scope S of

• the compound-statement of a function body or function try block,

the compund statement {body} of a lambda expression,

(since C++11)

• a substatement of a selection or iteration statement that is not itself a selection or iteration statement, or
• a handler of a function try block

potentially conflicts with a declaration whose target scope is the parent scope of S, the program is ill-formed.

if(int x = f())// declares “x”{// the if-block is a substatement of the if-statementint x;// error: redeclaration of “x”}else{// the else-block is also a substatement of the if-statementint x;// error: redeclaration of “x”}   void g(int i){externint i;// error: redeclaration of “i”}

Function parameter scope

Each parameter declarationP introduces a function parameter scope that includes P.

• If the declared parameter is of the parameter list of a function declaration:

• If the function declaration is a function definition, the scope introduced is extended to the end of the function definition.
• Otherwise (the function declaration is a function prototype), the scope introduced is extended to the end of the function declarator.
• In both cases, the scope does not include the locus of the function declaration.

If the declared parameter is of the parameter list of a lambda expression, the scope introduced is extended to the end of {body}.	(since C++11)
If the declared parameter is of the parameter list of a deduction guide, the scope introduced is extended to the end of that deduction guide.	(since C++17)
If the declared parameter is of the parameter list of a requires expression, the scope introduced is extended to the end of {requirement-seq}.	(since C++20)

int f(int n)// the declaration of the parameter “n”{// introduces a function parameter scope/* ... */}// the function parameter scope ends here

Lambda scope Each lambda expression introduces a lambda scope that starts immediately after [captures ] and extends to the end of {body}. The captures with initializers of a lambda expression E inhabit the lambda scope introduced by E. auto lambda =[x =1, y]()// this lambda expression introduces a lambda scope,{// it is the target scope of capture “x”/* ... */};// the lambda scope ends before the semicolon

(since C++14)

Namespace scope

Every namespace definition for a namespace N introduces a namespace scopeS that includes the declarations for every namespace definition for N.

For each non-friend redeclaration or specialization whose target scope is S or is contained by S, the following portions are also included in scope S:

• For a class (template) redeclaration or class template specialization, the portion after its class-head-name.
• For a enumeration redeclaration, the portion after its enum-head-name.
• For any other redeclaration or specialization, the portion after the unqualified-id or qualified-id of the declarator.

The global scope is the namespace scope of the global namespace.

namespace V // the namespace definition of “V”{// introduces a namespace scope “S”// the first part of scope “S” begins herevoid f();// the first part of scope “S” ends here}   void V::f()// the portion after “f” is also a part of scope “S”{void h();// declares V::h}// the second part of scope “S” ends here

Class scope

Each declaration of a class or class template C introduces a class scopeS that includes the member-specification of the class definition of C.

For each non-friend redeclaration or specialization whose target scope is S or is contained by S, the following portions are also included in scope S:

• For a class (template) redeclaration or class template specialization, the portion after its class-head-name.
• For a enumeration redeclaration, the portion after its enum-head-name.
• For any other redeclaration or specialization, the portion after the unqualified-id or qualified-id of the declarator.

class C // the class definition of “C”{// introduces a class scope “S”// the first part of scope “S” begins herevoid f();// the first part of scope “S” ends here}   void C::f()// the portion after “f” is also a part of scope “S”{/* ... */}// the second part of scope “S” ends here

Enumeration scope

Each declaration of an enumeration E introduces an enumeration scope that includes the enumerator-list of the non-opaque(since C++11)enumeration declaration of E (if present).

enumclass E // the enumeration declaration of “E”{// introduces an enumeration scope “S”// scope “S” begins here e1, e2, e3 // scope “S” ends here}

Template parameter scope

Each template template parameter introduces a template parameter scope that includes the entire template parameter list and the require clauses(since C++20) of that template template parameter.

Each template declaration D introduces a template parameter scopeS that extends from the beginning of the template parameter list of D to the end of D. Any declaration outside the template parameter list that would inhabit S instead inhabits the same scope as D.

Only template parameters belong to a template parameter scope, and only template parameter scopes have a template parameter scope as a parent scope.

// the class template declaration of “X”// introduces a template parameter scope “S1”template<// scope “S1” begins heretemplate// the template template parameter “T”// introduces another template parameter scope “S2”<typename T1 typename T2 > requires std::convertible_from<T1, T2>// scope “S2” ends hereclass T, typename U >class X;// scope “S1” ends before the semicolon   namespace N {template<typename T>using A =struct X;// “X” inhabits the same scope as template// declaration, namely the scope of “N”}

Contract-assertion scope Each contract assertionC introduces a contract-assertion scope that includes C. If a postcondition assertion has an identifier which is not name-independent, and the postcondition assertion is associated with a function funcpotentially conflicts with a declaration D whose target scope is one of the following scopes, the program is ill-formed: The function parameter scope of func. If D is associated with a lambda expression, the nearest enclosing lambda scope of the precondition assertion.

(since C++26)

Point of declaration

In general, a name is visible after the locus of its first declaration, which is located as follows.

The locus of a name declared in a simple declaration is immediately after that name's declarator and before its initializer, if any.

int x =32;// outer x is in scope   {int x = x;// inner x is in scope before the initializer (= x)// this does not initialize inner x with the value of outer x (32),// this initializes inner x with its own (indeterminate) value}   std::function<int(int)> f =[&](int n){return n >1? n * f(n -1): n;};// the name of the function f is in scope in the lambda and can// be correctly captured by reference, giving a recursive function

constint x =2;// outer x is in scope   {int x[x]={};// inner x is in scope before the initializer (= {}),// but after the declarator (x[x])// in the declarator, outer x is still in scope// this declares an array of 2 int}

The locus of a class or class template declaration is immediately after the identifier that names the class (or the template-id that names the template specialization) in its class-head. The class or class template name is already in scope in the list of base classes.

struct S:std::enable_shared_from_this<S>{};// S is in scope at the colon

The locus of enum specifier or opaque enum declaration(since C++11) is immediately after the identifier that names the enumeration.

enum E :int// E is in scope at the colon{ A = sizeof(E)};

The locus of a type alias or alias template declaration is immediately after the type-id to which the alias refers.

using T =int;// outer T is in scope at the semicolon   {using T = T*;// inner T is in scope at the semicolon,// outer T is still in scope before the semicolon// same as T = int*}

The locus for a declarator in a using declaration that does not name a constructor is immediately after the declarator.

template<int N>class Base {protected:staticconstint next = N +1;staticconstint value = N;};   struct Derived: Base<0>, Base<1>, Base<2>{using Base<0>::next, // next is in scope at the comma Base<next>::value;// Derived::value is 1};

The locus of an enumerator is immediately after its definition (not before the initializer as it is for variables).

constint x =12;   {enum{ x = x +1, // enumerator x is in scope at the comma,// outer x is in scope before the comma,// enumerator x is initialized to 13 y = x +1// y is initialized to 14};}

The locus for an injected-class-name is immediately following the opening brace of its class (or class template) definition.

template<typename T>struct Array //  : std::enable_shared_from_this<Array> // error: the injected class name is not in scope:std::enable_shared_from_this< Array<T>>// OK: the template-name Array is in scope{// the injected class name Array is now in scope as if a public member name Array* p;// pointer to Array<T>};

The locus of the implicit declaration for a function-local predefined variable __func__ is immediately before the function body of a function definition.

(since C++11)

The locus of a structured binding declaration is immediately after the identifier-list, but structured binding initializers are prohibited from referring to any of the names being declared.

(since C++17)

The locus of the variable or the structured bindings(since C++17) declared in the range-declaration of a range-for loop is immediately after the range-expression. std::vector<int> x;   for(auto x : x)// vector x is in scope before the closing parenthesis,// auto x is in scope at the closing parenthesis{// the auto x is in scope}

(since C++11)

The locus of a template parameter is immediately after its complete template parameter (including the optional default argument).

typedefunsignedchar T;   template<class T = T, // template parameter T is in scope at the comma,// typedef name of unsigned char is in scope before the comma T // template parameter T is in scope N =0>struct A {};

The locus of a postcondition assertion with an identifier is immediately after its :.

(since C++26)

The locus of a concept definition is immediately after the concept name, but concept definitions are prohibited from referring to the concept name being declared.

(since C++20)

The locus of a named namespace definition is immediately after the namespace name.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 2793	C++98	an extern declaration in a block scope could conflict with another declaration in the parent scope	prohibited

References

• C++23 standard (ISO/IEC 14882:2024):

• 6.4 Scope [basic.scope]

• C++20 standard (ISO/IEC 14882:2020):

• 6.4 Scope [basic.scope]

• C++17 standard (ISO/IEC 14882:2017):

• 6.3 Scope [basic.scope]

• C++14 standard (ISO/IEC 14882:2014):

• 3.3 Scope [basic.scope]

• C++11 standard (ISO/IEC 14882:2011):

• 3.3 Scope [basic.scope]

• C++98 standard (ISO/IEC 14882:1998):

• 3.3 Declarative regions and scopes [basic.scope]

See also

C documentation for Scope