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テキスト

执行明确的类型转换。

句法

( type-name ) expression



其中

类型名

-

无论是 void 类型还是任何标量类型

表达

-

标量类型的任何表达式(除非类型名称是无效的,在这种情况下,它可以是任何东西)

说明

如果 type-name 是void,那么表达式会计算其副作用,并将其返回值作为表达式语句放弃,与将表达式单独使用时相同。

否则,如果 type-name 恰好是表达式的类型,则不做任何事情(除非表达式具有浮动类型,并且表示的范围和精度比其类型指示的大 - 参见下文)。

否则,表达式的值将转换为由 type-name 命名的类型,如下所示:

每个隐式转换就像是通过赋值一样是允许的。

除了隐式转换之外,还允许进行以下转换:

  • 任何整数都可以转换为任何指针类型。除了空指针常量NULL(不需要强制转换)之外,结果是实现定义的,可能未正确对齐,可能不指向引用类型的对象,并且可能是陷阱表示。

  • 任何指针类型都可以转换为任何整数类型。结果是实现定义的,即使对于空指针值(它们不一定会导致值为零)。如果结果不能在目标类型中表示,则行为是未定义的(无符号整数不能对来自指针的强制执行模运算)

  • 任何指向对象的指针都可以转换为任何其他指向对象的指针。如果该值未针对目标类型正确对齐,则行为未定义。否则,如果该值被转换回原始类型,则它与原始值相等。如果指向对象的指针转换为指向任何字符类型的指针,则结果指向该对象的最低字节,并且可以递增至目标类型的大小(换言之,可用于检查对象表示或制作memcpy或通过复制memmove)。

  • 任何指向函数的指针都可以转换为指向任何其他函数类型的指针。如果生成的指针转换回原始类型,则它与原始值相等。如果转换的指针用于进行函数调用,则行为是未定义的(除非函数类型兼容)

  • 在指针(对象或函数)之间进行转换时,如果原始值是其类型的空指针值,则结果是目标类型的正确空指针值。

无论如何(在执行隐式转换时以及在同一类型转换中),如果 expression 和 type-name 是浮点类型,并且表达式的范围和精度比其类型所指示FLT_EVAL_METHOD的范围和精度要高(参见范围和精度被删除)关闭以匹配目标类型。

转换表达式的值类别始终是非左值。

笔记

因为const,volatile,restrict 和 atomic 限定符只对左值有影响,所以对 cvr 限定或原子类型的转换完全等同于转换为相应的非限定类型。

无效的转换有时可以用来消除关于未使用结果的编译器警告。

未列在这里的转换不被允许。尤其是,

  • 指针和浮点类型之间没有转换

  • 在指向函数的指针和指向对象的指针之间没有转换(包括void*

  • 在指向函数和整数的指针之间没有转换

#include <stdio.h>
 int main(void){    // examining object representation is a legitimate use of cast
    double d = 3.14;    printf("The double %.2f(%a) is: ", d, d);    for(size_t n = 0; n < sizeof d; ++n)        printf("0x%02x ", ((unsigned char*)&d)[n]); 
    // edge cases
    struct S {int x;} s;//    (struct S)s; // error; not a scalar type                   // even though casting to the same type does nothing    (void)s; // okay to cast any type to void}

可能的输出:

The double 3.14(0x1.91eb851eb851fp+1) is: 0x1f 0x85 0xeb 0x51 0xb8 0x1e 0x09 0x40

参考

  • C11 standard (ISO/IEC 9899:2011):

    • 6.5.4 Cast operators (p: 91)

  • C99 standard (ISO/IEC 9899:1999):

    • 6.5.4 Cast operators (p: 81)

  • C89/C90 standard (ISO/IEC 9899:1990):

    • 3.3.4 Cast operators

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