Conditionally executes a statement repeatedly, where the statement does not need to manage the loop condition.

[edit]Syntax

[edit]Condition

Acondition can either be anexpression or asimple declaration.

• If it can be syntactically resolved as an expression, it is treated as an expression. Otherwise, it is treated as a declaration that is not a structured binding declaration(since C++26).

When control reaches condition, the condition will yield a value, which is used to determine whetherstatement will be executed.

[edit]Expression

Ifcondition is an expression, the value it yields is the the value of the expression contextually converted tobool. If that conversion is ill-formed, the program is ill-formed.

[edit]Declaration

Ifcondition is a simple declaration, the value it yields is the value of the decision variable (see below) contextually converted tobool. If that conversion is ill-formed, the program is ill-formed.

[edit]Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• The declarator cannot specify afunction or anarray.
• Thetype specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers andconstexpr, and it(since C++11) cannot define aclass orenumeration.

The decision variable of the declaration is the declared variable.

[edit]Explanation

Afor statement equivalent to:

Except that

• The scope ofinit-statement and the scope ofcondition are the same.
• The scope ofstatement and the scope ofexpression are disjoint and nested within the scope ofinit-statement andcondition.
• Executing acontinue statement instatement will evaluateexpression.
• Emptycondition is equivalent totrue.

If the loop needs to be terminated withinstatement, abreak statement can be used as terminating statement.

If the current iteration needs to be terminated withinstatement, acontinue statement can be used as shortcut.

[edit]Notes

As is the case withwhile loop, ifstatement is not a compound statement, the scope of variables declared in it is limited to the loop body as if it was a compound statement.

for(;;)int n;// n goes out of scope

As part of the C++forward progress guarantee, the behavior isundefined if a loop that is not atrivial infinite loop(since C++26) withoutobservable behavior does not terminate. Compilers are permitted to remove such loops.

While in C names declared in the scope ofinit-statement andcondition can be shadowed in the scope ofstatement, it is forbidden in C++:

for(int i=0;;){long i=1;// valid C, invalid C++// ...}

[edit]Keywords

for

[edit]Example

#include <iostream>#include <vector> int main(){std::cout<<"1) typical loop with a single statement as the body:\n";for(int i=0; i<10;++i)std::cout<< i<<' '; std::cout<<"\n\n""2) init-statement can declare multiple names, as\n""long as they can use the same decl-specifier-seq:\n";for(int i=0,*p=&i; i<9; i+=2)std::cout<< i<<':'<<*p<<' '; std::cout<<"\n\n""3) condition may be a declaration:\n";char cstr[]="Hello";for(int n=0;char c= cstr[n];++n)std::cout<< c; std::cout<<"\n\n""4) init-statement can use the auto type specifier:\n";std::vector<int> v={3,1,4,1,5,9};for(auto iter= v.begin(); iter!= v.end();++iter)std::cout<<*iter<<' '; std::cout<<"\n\n""5) init-statement can be an expression:\n";int n=0;for(std::cout<<"Loop start\n";std::cout<<"Loop test\n";std::cout<<"Iteration "<<++n<<'\n'){if(n>1)break;} std::cout<<"\n""6) constructors and destructors of objects created\n""in the loop's body are called per each iteration:\n";struct S{        S(int x,int y){std::cout<<"S::S("<< x<<", "<< y<<"); ";}        ~S(){std::cout<<"S::~S()\n";}};for(int i{0}, j{5}; i< j;++i,--j)        S s{i, j}; std::cout<<"\n""7) init-statement can use structured bindings:\n";long arr[]{1,3,7};for(auto[i, j, k]= arr; i+ j< k;++i)std::cout<< i+ j<<' ';std::cout<<'\n';}

1) typical loop with a single statement as the body:0 1 2 3 4 5 6 7 8 9 2) init-statement can declare multiple names, aslong as they can use the same decl-specifier-seq:0:0 2:2 4:4 6:6 8:8 3) condition may be a declaration:Hello 4) init-statement can use the auto type specifier:3 1 4 1 5 9 5) init-statement can be an expression:Loop startLoop testIteration 1Loop testIteration 2Loop test 6) constructors and destructors of objects createdin the loop's body are called per each iteration:S::S(0, 5); S::~S()S::S(1, 4); S::~S()S::S(2, 3); S::~S() 7) init-statement can use structured bindings:4 5 6

[edit]See also

• In other languages

• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文