Statements

 

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	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++

 

Statements

Labels

label : statement

Expression statements

expression ;

Compound statements

{ statement... }

Selection statements

if

switch

Iteration statements

while

do while

for

range for (C++11)

Jump statements

break

continue

return

goto

Declaration statements

declaration ;

Try blocks

try block

Transactional memory

synchronized, atomic_commit, etc (TM TS)

 

Statements are fragments of the C++ program that are executed in sequence. The body of any function is a sequence of statements. For example:

int main()
{
    int n = 1;                        // declaration statement
    n = n + 1;                        // expression statement
    std::cout << "n = " << n << '\n'; // expression statement
    return 0;                         // return statement
}

C++ includes the following types of statements:

1) labeled statements;

2) expression statements;

3) compound statements;

4) selection statements;

5) iteration statements;

6) jump statements;

7) declaration statements;

8) try blocks;

9) atomic and synchronized blocks (TM TS).

Labeled statements

A labeled statement labels a statement for control flow purposes.

label statement

label	-	the label applied to the statement (defined below)
statement	-	the statement which the label applies to, it can be a labeled statement itself, allowing multiple labels

Labels

label is defined as

attr (optional) identifier :	(1)
attr (optional) case constexpr :	(2)
attr (optional) default:	(3)

1) target for goto;

2) case label in a switch statement;

3) default label in a switch statement.

An attribute sequence attr may appear just at the beginning of the label (in which case it applies to the label), or just before any statement itself, in which case it applies to the entire statement.

(since C++11)

A label with an identifier declared inside a function matches all goto statements with the same identifier in that function, in all nested blocks, before and after its own declaration.

Two labels in a function must not have the same identifier.

Besides being added to a statement, labels can also be used anywhere in compound statements.

(since C++23)

Labels are not found by unqualified lookup: a label can have the same name as any other entity in the program.

void f()
{
    {
        goto label; // label in scope even though declared later
        label:      // label can appear at the end of a block standalone since C++23
    }
    goto label; // label ignores block scope
}
 
void g()
{
    goto label; // error: label not in scope in g()
}

Control-flow-limited statements

The following statements are control-flow-limited statements :

• The compound-statement of a try block.
• The compound-statement of a handler.

All substatements of a constexpr if statement.	(since C++17)
All substatements of a consteval if statement.	(since C++23)

For each control-flow-limited statement S:

• All goto target labels delcared in S can only be referred to by statements in S.
• Each case or default label appearing within S can only be associated with a switch statement within S.

Expression statements

An expression statement is an expression followed by a semicolon.

attr (optional) expression (optional) ;

attr	-	(since C++11) optional sequence of any number of attributes
expression	-	an expression

Most statements in a typical C++ program are expression statements, such as assignments or function calls.

An expression statement without an expression is called a null statement. It is often used to provide an empty body to a for or while loop. It can also be used to carry a label in the end of a compound statement.(until C++23)

Compound statements

A compound statement or block groups a sequence of statements into a single statement.

attr (optional) { statement... (optional) label... (optional)(since C++23) }

When one statement is expected, but multiple statements need to be executed in sequence (for example, in an if statement or a loop), a compound statement may be used:

if (x > 5)          // start of if statement
{                   // start of block
    int n = 1;      // declaration statement
    std::cout << n; // expression statement
}                   // end of block, end of if statement

Each compound statement introduces its own block scope; variables declared inside a block are destroyed at the closing brace in reverse order:

int main()
{ // start of outer block
    {                                // start of inner block
        std::ofstream f("test.txt"); // declaration statement
        f << "abc\n";                // expression statement
    }                                // end of inner block, f is flushed and closed
    std::ifstream f("test.txt"); // declaration statement
    std::string str;             // declaration statement
    f >> str;                    // expression statement
} // end of outer block, str is destroyed, f is closed

A label at the end of a compound statement is treated as if it were followed by a null statement.

(since C++23)

Selection statements

A selection statement chooses between one of several control flows.

attr (optional) if constexpr(optional) ( init-statement (optional) condition ) statement	(1)
attr (optional) if constexpr(optional) ( init-statement (optional) condition ) statement else statement	(2)
attr (optional) switch ( init-statement (optional) condition ) statement	(3)
attr (optional) if !(optional) consteval compound-statement	(4)	(since C++23)
attr (optional) if !(optional) consteval compound-statement else statement	(5)	(since C++23)

1) if statement;

2) if statement with an else clause;

3) switch statement;

4) consteval if statement;

5) consteval if statement with an else clause.

Iteration statements

An iteration statement repeatedly executes some code.

attr (optional) while ( condition ) statement	(1)
attr (optional) do statement while ( expression ) ;	(2)
attr (optional) for ( init-statement condition (optional) ; expression (optional) ) statement	(3)
attr (optional) for ( init-statement (optional)(since C++20) for-range-decl : for-range-init ) statement	(4)	(since C++11)

1) while loop;

2) do-while loop;

3) for loop;

4) range for loop.

Jump statements

A jump statement unconditionally transfers control flow.

attr (optional) break;	(1)
attr (optional) continue;	(2)
attr (optional) return expression (optional) ;	(3)
attr (optional) return braced-init-list ;	(4)	(since C++11)
attr (optional) goto identifier ;	(5)

1) break statement;

2) continue statement;

3) return statement with an optional expression;

4) return statement using list initialization;

5) goto statement.

Note: for all jump statements, transfer out of a loop, out of a block, or back past an initialized variable with automatic storage duration involves the destruction of objects with automatic storage duration that are in scope at the point transferred from but not at the point transferred to. If multiple objects were initialized, the order of destruction is the opposite of the order of initialization.

Declaration statements

A declaration statement introduces one or more identifiers into a block.

block-declaration	(1)

1) See Declarations and Initialization for details.

try blocks

A try block catches exceptions thrown when executing other statements.

attr (optional) try compound-statement handler-sequence	(1)

1) See try block for details.

Atomic and synchronized blocks An atomic and synchronized block provides transactional memory. synchronized compound-statement (1) (TM TS) atomic_noexcept compound-statement (2) (TM TS) atomic_cancel compound-statement (3) (TM TS) atomic_commit compound-statement (4) (TM TS) 1) synchronized block, executed in single total order with all synchronized blocks; 2) atomic block that aborts on exceptions; 3) atomic block that rolls back on exceptions; 4) atomic block that commits on exceptions.

(TM TS)

See also

C documentation for Statements