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Accepted
Егор Филатов
Asked:2020-08-27 19:47:30 +0000 UTC2020-08-27 19:47:30 +0000 UTC

函数参数中`int()`和`int(*)()`的区别

  • 772

有什么区别：

int fun(int ()) {
    return 1;
}

和

int fun(int (*)()) {
    return 1;
}
c++

3 个回答

  1. Best Answer

    您可以在以下工具的帮助下自行检查是否存在差异以及差异：编译器和nm. 当您尝试编译同时包含两个定义的程序（或 lib）时，namespace您将收到有关重新定义函数的错误。如果你先用第一个编译，然后用第二个选项编译，然后用 help 看看发生了什么nm，你会看到下面的图片（没有修改）：

    0000000000004028 B __bss_start
0000000000004028 b completed.7325
                 w __cxa_finalize@@GLIBC_2.2.5
0000000000004018 D __data_start
0000000000004018 W data_start
0000000000001070 t deregister_tm_clones
00000000000010e0 t __do_global_dtors_aux
0000000000003df0 t __do_global_dtors_aux_fini_array_entry
0000000000004020 D __dso_handle
0000000000003df8 d _DYNAMIC
0000000000004028 D _edata
0000000000004030 B _end
00000000000011a4 T _fini
0000000000001120 t frame_dummy
0000000000003de8 t __frame_dummy_init_array_entry
000000000000216c r __FRAME_END__
0000000000004000 d _GLOBAL_OFFSET_TABLE_
                 w __gmon_start__
0000000000002004 r __GNU_EH_FRAME_HDR
0000000000001000 t _init
0000000000003df0 t __init_array_end
0000000000003de8 t __init_array_start
0000000000002000 R _IO_stdin_used
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
00000000000011a0 T __libc_csu_fini
0000000000001140 T __libc_csu_init
                 U __libc_start_main@@GLIBC_2.2.5
0000000000001134 T main
00000000000010a0 t register_tm_clones
0000000000001040 T _start
0000000000004028 D __TMC_END__
0000000000001125 T fun(int (*)())

    0000000000004028 B __bss_start
0000000000004028 b completed.7325
                 w __cxa_finalize@@GLIBC_2.2.5
0000000000004018 D __data_start
0000000000004018 W data_start
0000000000001070 t deregister_tm_clones
00000000000010e0 t __do_global_dtors_aux
0000000000003df0 t __do_global_dtors_aux_fini_array_entry
0000000000004020 D __dso_handle
0000000000003df8 d _DYNAMIC
0000000000004028 D _edata
0000000000004030 B _end
00000000000011a4 T _fini
0000000000001120 t frame_dummy
0000000000003de8 t __frame_dummy_init_array_entry
000000000000216c r __FRAME_END__
0000000000004000 d _GLOBAL_OFFSET_TABLE_
                 w __gmon_start__
0000000000002004 r __GNU_EH_FRAME_HDR
0000000000001000 t _init
0000000000003df0 t __init_array_end
0000000000003de8 t __init_array_start
0000000000002000 R _IO_stdin_used
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
00000000000011a0 T __libc_csu_fini
0000000000001140 T __libc_csu_init
                 U __libc_start_main@@GLIBC_2.2.5
0000000000001134 T main
00000000000010a0 t register_tm_clones
0000000000001040 T _start
0000000000004028 D __TMC_END__
0000000000001125 T fun(int (*)())

    Tobish 第一个选项和第二个选项只是同一条目的两个版本

    • 15

  2. 在推断函数参数的类型时，任何函数类型都会被指向函数的指针替换。[dcl.fct]/5：

    [...] 在确定每个参数的类型后，将任何类型为“array of T”或函数类型T的参数调整为“pointer to T”。[...]

    因此，int ()函数参数中的类型实际上表示类型int (*)()。

    示例：

    #include <iostream>
#include <type_traits>

using std::cout;
using std::endl;

int f(int ptr())
{
    cout << "f: " << std::is_same<int (*)(), decltype(ptr)>::value << endl; //true
    return 0;
}

int g()
{
    return 0;
}

int main()
{
    cout << "main: " << std::is_same<int (*)(), decltype(g)>::value << endl; //false
    f(nullptr);
}
    • 12

  3. 函数参数中没有任何内容。但是伏击可以在不在函数参数中使用时使用 - https://godbolt.org/z/6xzzeW。

    #include <iostream>

using namespace std;


using Callback1_t = int (double);
using Callback2_t = int (*)(double);


int func1(Callback1_t fn1)
{
    fn1(1.0);
    return 1;
}


int func2(Callback2_t fn2)
{
    func1(fn2);
    fn2(2.0);
    return 2;
}


class A {
    Callback1_t fn1_;
    Callback2_t fn2_;
public:
    A(Callback1_t fn1, Callback2_t fn2) : fn1_(fn1), fn2_(fn2) {}
};


int main(int argc, char* argv[])
{
    return 0;
}

    <source>: In constructor 'A::A(int (*)(double), Callback2_t)':
<source>:29:43: error: class 'A' does not have any field named 'fn1_'
   29 |     A(Callback1_t fn1, Callback2_t fn2) : fn1_(fn1), fn2_(fn2) {}
      |                                           ^~~~
    • 6

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