if statement

IF语句

有条件地执行另一条语句。

用于需要根据运行时或编译时条件执行代码的地方。

句法

attr(optional) if ( condition ) statement-true (1) attr(optional) if ( condition ) statement-true else statement-false (2)	attr(optional) if ( condition ) statement-true	(1)	attr(optional) if ( condition ) statement-true else statement-false	(2)	(until C++17)
attr(optional) if ( condition ) statement-true	(1)
attr(optional) if ( condition ) statement-true else statement-false	(2)
attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true (1) attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true else statement-false (2)	attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true	(1)	attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true else statement-false	(2)	(since C++17)
attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true	(1)
attr(optional) if constexpr(optional) ( init-statement(optional) condition ) statement-true else statement-false	(2)

attr(C++11)	-	any number of attributes
condition	-	one of expression which is contextually convertible to bool declaration of a single non-array variable with a brace-or-equals initializer.
init-statement(C++17)	-	either an expression statement (which may be a null statement ";") a simple declaration, typically a declaration of a variable with initializer, but it may declare arbitrary many variables or be a decomposition declaration Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.
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

表达那就是上下文可转换下流

声明具有大括号或相等值的单个非数组变量的初始化器...

 init-statement(C++17)   -   either

安表达式语句%28，这可能是空语句“;“%29

阿简单声明，通常是带有初始化器的变量的声明，但它可以声明任意多个变量，也可以声明为分解声明。

注意，任何init语句都必须以分号结尾。;，这就是为什么它经常被非正式地描述为一个表达式或一个声明，后面跟着分号。陈述-真实-任何陈述%28通常是一个复合语句%29，如果条件计算为true陈述-假-任何陈述%28通常是一个复合语句%29，如果条件计算为false

解释

如果条件产生true转换为bool，语句-true被执行。

如果if语句的其他部分存在并且条件产生false转换为bool，语句-false被执行。

在if语句%28的第二种形式中，包括Other%29在内的if语句-true也是if语句，那么该内部if语句也必须包含%28，换句话说，在嵌套的if-语句中，如果%27T有一个其他的%29，则该语句与最接近的语句相关联。

二次

#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 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
}

二次

产出：

二次

2 is not greater than 2
2 > 1 and 1 <= 2
df()

二次

If init-statement is used, the if statement is equivalent to. { init_statement if constexpr(optional) ( condition ) statement-true } or. { init_statement if constexpr(optional) ( condition ) statement-true else statement-false } Except that names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a declaration) are in the same scope, which is also the scope of both statements. std::map m; std::mutex mx; extern bool shared_flag; // guarded by mx int demo() { if (auto it = m.find(10 it != m.end()) { return it->size( } if (char buf10; std::fgets(buf, 10, stdin)) { m0 += buf; } if (std::lock_guard lock(mx shared_flag) { unsafe_ping( shared_flag = false; } if (int s; int count = ReadBytesWithSignal(&s)) { publish(count raise(s } if (auto keywords = {"if", "for", "while"}; std::any_of(keywords.begin(), keywords.end(), &s { return s == kw; })) { std::cerr << "Token must not be a keyword\n" } }	{ init_statement if constexpr(optional) ( condition ) statement-true }			{ init_statement if constexpr(optional) ( condition ) statement-true else statement-false }			(since C++17)
{ init_statement if constexpr(optional) ( condition ) statement-true }
{ init_statement if constexpr(optional) ( condition ) statement-true else statement-false }

Constexpr If The statement that begins with if constexpr is known as the constexpr if statement. In a constexpr if statement, the value of condition must be a contextually converted constant expression of type bool. If the value is true, then statement-false is discarded (if present), otherwise, statement-true is discarded. The return statements in a discarded statement do not participate in function return type deduction: template auto get_value(T t) { if constexpr (std::is_pointer_v) return *t; // deduces return type to int for T = int* else return t; // deduces return type to int for T = int } The discarded statement can odr-use a variable that is not defined. extern int x; // no definition of x required int f() { if constexpr (true) return 0; else if (x) return x; else return -x; } If a constexpr if statement appears inside a templated entity, and if condition is not value-dependent after instantiation, the discarded statement is not instantiated when the enclosing template is instantiated . template void g(T&& p, Rest&& ...rs) { // ... handle p if constexpr (sizeof...(rs) > 0) g(rs... // never instantiated with an empty argument list. } Note: an example where the condition remains value-dependent after instantiation is a nested template, e.g. template void g() { auto lm = { if constexpr (sizeof(T) == 1 && sizeof p == 1) { // this condition remains value-dependent after instantiation of g } }; } Note: the discarded statement can't be ill-formed for every possible specialization: template void f() { if constexpr (std::is_arithmetic_v) // ... else static_assert(false, "Must be arithmetic" // ill-formed: invalid for every T } The common workaround for such a catch-all statement is a type-dependent expression that is always false: template struct dependent_false : std::false_type {}; template void f() { if constexpr (std::is_arithmetic_v) // ... else static_assert(dependent_false::value, "Must be arithmetic" // ok } Labels (goto targets, case labels, and default:) appearing in a substatement of a constexpr if can only be referenced (by switch or goto) in the same substatement.

(since C++17)

注记

IF语句[医]真实或陈述[医]false不是一个复合语句，它被当作是：

二次

if (x)
    int i;
// i is no longer in scope

二次

和。

二次

if (x) {
    int i;
} // i is no longer in scope

二次

按条件引入的名称范围(如果是声明)是两个语句%27的合并范围：

二次

if (int x = f()) {
    int x; // error: redeclaration of x
} else {
    int x; // error: redeclaration of x
}

二次

If statement-true is entered by goto or longjmp, statement_false is not executed.	(since C++14)
Switch and goto are not allowed to jump into a branch of constexpr if statement.	(since C++17)

关键词

if，，，else，，，constexpr...

另见

c IF语句的文件

*。

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