Scope

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Basic Concepts
 

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

S always appear in another scope, which thereby contains S.

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      T
int 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

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

Block-scope extern declarations target a larger enclosing scope, but bind a name in their immediate scope.

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

(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-statement
    int x;        // error: redeclaration of “x”
}
else
{ // the else-block is also a substatement of the if-statement
    int x;        // error: redeclaration of “x”
}
 
void g(int i)
{
    extern int i; // error: redeclaration of “i”
}

Function parameter scope

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

  • 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 scope S 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 here
    void 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 scope S 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 here
    void 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). 

enum class 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 scope S 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. 

// the class template declaration of “X”
// introduces a template parameter scope “S1”
template
<
    // scope “S1” begins here
    template // the template template parameter “T”
             // introduces another template parameter scope “S2”
    <
        typename T1
        typename T2
    > requires std::convertible_from<T1, T2> // scope “S2” ends here
    class T,
    typename U
>
class X; // scope “S1” ends before the semicolon

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
const int 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:
    static const int next = N + 1;
    static const int 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). 

const int 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). 

typedef unsigned char 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 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.

This section is incomplete
Reason: remainder of [basic.scope.pdecl]

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
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