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Numerics abs acos acosf acosh acoshf acoshl acosl asin asinf asinh asinhf asinhl asinl atan atan2 atan2f atan2l atanf atanh atanhf atanhl atanl cabs cabsf cabsl cacos cacosf cacosh cacoshf cacoshl cacosl carg cargf cargl casin casinf casinh casinhf casinhl casinl catan catanf catanh catanhf catanhl catanl cbrt cbrtf cbrtl ccos ccosf ccosh ccoshf ccoshl ccosl ceil ceilf ceill cexp cexpf cexpl cimag cimagf cimagl clog clogf clogl CMPLX CMPLXF CMPLXL Common mathematical functions complex Complex number arithmetic conj conjf conjl copysign copysignf copysignl cos cosf cosh coshf coshl cosl cpow cpowf cpowl cproj cprojf cprojl creal crealf creall csin csinf csinh csinhf csinhl csinl csqrt csqrtf csqrtl ctan ctanf ctanh ctanhf ctanhl ctanl div double_t erf erfc erfcf erfcl erff erfl exp exp2 exp2f exp2l expf expl expm1 expm1f expm1l fabs fabsf fabsl fdim feclearexcept fegetenv fegetexceptflag fegetround feholdexcept feraiseexcept fesetenv fesetexceptflag fesetround fetestexcept feupdateenv 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number generation rand RAND_MAX remainder remainderf remainderl remquo remquof remquol rint rintf rintl round roundf roundl scalbln scalblnf scalblnl scalbn scalbnf scalbnl signbit sin sinf sinh sinhf sinhl sinl sqrt sqrtf sqrtl srand tan tanf tanh tanhf tanhl tanl tgamma tgammaf tgammal trunc truncf truncl Type-generic math _Complex_I _Imaginary_I Program support abort atexit at_quick_exit exit EXIT_FAILURE EXIT_SUCCESS getenv getenv_s jmp_buf longjmp Program support utilities quick_exit raise setjmp SIGABRT SIGFPE SIGILL SIGINT signal SIGSEGV SIGTERM sig_atomic_t SIG_DFL SIG_ERR SIG_IGN system _Exit Strings atof atoi atol atoll btowc c16rtomb c32rtomb char16_t char32_t isalnum isalpha isblank iscntrl isdigit isgraph islower isprint ispunct isspace isupper iswalnum iswalpha iswblank iswcntrl iswctype iswdigit iswgraph iswlower iswprint iswpunct iswspace iswupper iswxdigit isxdigit mblen mbrlen mbrtoc16 mbrtoc32 mbrtowc mbsinit mbsrtowcs mbsrtowcs_s mbstate_t mbstowcs mbstowcs_s mbtowc memchr memcmp memcpy memcpy_s memmove memmove_s memset memset_s Null-terminated byte strings Null-terminated multibyte strings Null-terminated wide strings strcat strcat_s strchr strcmp strcoll strcpy strcpy_s strcspn strerror strerrorlen_s strerror_s Strings library strlen strncat Thread support call_once cnd_broadcast cnd_destroy cnd_init cnd_signal cnd_timedwait cnd_wait mtx_destroy mtx_init mtx_lock mtx_plain mtx_recursive mtx_timed mtx_timedlock mtx_trylock mtx_unlock once_flag ONCE_FLAG_INIT thrd_busy thrd_create thrd_current thrd_detach thrd_equal thrd_error thrd_exit thrd_join thrd_nomem thrd_sleep thrd_success thrd_timedout thrd_yield Thread support library thread_local tss_create tss_delete TSS_DTOR_ITERATIONS tss_get tss_set Type support Boolean type support library Fixed width integer types FLT_EVAL_METHOD FLT_ROUNDS max_align_t NULL Numeric limits offsetof ptrdiff_t size_t Type support Variadic functions Variadic functions va_arg va_copy va_end va_list va_start
文字

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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