if statement
Conditionally executes another statement.
Used where code needs to be executed based on a run-time condition.
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[edit] Syntax
attr(optional) if ( condition ) statement_true
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attr(optional) if ( condition ) statement_true else statement_false
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attr(C++11) | - | any number of attributes |
condition | - | one of
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statement_true | - | any statement (often a compound statement), which is executed if condition evaluates to true |
statement_false | - | any statement (often a compound statement), which is executed if condition evaluates to false |
[edit] Explanation
If the condition yields true after conversion to bool, statement_true is executed.
If the else part of the if statement is present and condition yields false after conversion to bool, statement_false is executed.
In the second form of if statement (the one including else), if statement_true is also an if statement then that inner if statement must contain an else part as well (in other words, in nested if-statements, the else is associated with the closest if that doesn't have an else)
[edit] Notes
If statement_true or statement_false is not a compound statement, it is treated as if it were:
if(x) int i; // i is no longer in scope
is the same as
if(x) { int i; } // i is no longer in scope
The scope of the name introduced by condition, if it is a declaration, is the same as the scope of the body of the statements:
if (int x = f()) { int x; // error: redeclaration of x } else { int x; // error: redeclaration of x }
If statement_true is entered by goto or longjmp, statement_false is not executed. |
(since C++14) |
[edit] Keywords
[edit] Example
The following example demonstrates several usage cases of the if
statement
#include <iostream> int main() { // simple if-statement with an else clause int i = 2; if (i > 2) { std::cout << i << " is greater than 2\n"; } else { std::cout << i << " is not greater than 2\n"; } // nested if-statement int j = 1; if (i > 1) if(j > 2) std::cout << i << " > 1 and " << j << " > 2\n"; else // this else is part of if(j>2), not part of if(i>1) std::cout << i << " > 1 and " << j << " <= 2\n"; // declarations can be used as conditions with dynamic_cast struct Base { virtual ~Base() {} }; struct Derived : Base { void df() { std::cout << "df()\n"; } }; Base* bp1 = new Base; Base* bp2 = new Derived; if(Derived* p = dynamic_cast<Derived*>(bp1)) // cast fails, returns NULL p->df(); // not executed if(auto p = dynamic_cast<Derived*>(bp2)) // cast succeeds p->df(); // executed }
Output:
2 is not greater than 2 2 > 1 and 1 <= 2 df()
[edit] See Also
C documentation for if statement
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