cpp：' data_type * '被视为一个数据类型

一、结论

　　1、data_type *，它被视为一个数据类型；

　　2、data_type *，用于定义指针变量；

　　3、举例：

　　　　int * pt = nullptr;　　// int* 被视为一个数据类型；

  1 [root@rockylinux tmp]# uname -a
  2 Linux rockylinux 4.18.0-372.19.1.el8_6.x86_64 #1 SMP Tue Aug 2 16:19:42 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
  3 [root@rockylinux tmp]#
  4 [root@rockylinux tmp]# g++ --version
  5 g++ (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10)
  6 Copyright (C) 2018 Free Software Foundation, Inc.
  7 This is free software; see the source for copying conditions.  There is NO
  8 warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  9 
 10 [root@rockylinux tmp]#
 11 [root@rockylinux tmp]# cat data_pointer.cpp
 12 /*
 13  *
 14  *      file_name = data_pointer.cpp
 15  *      date = 2022-08-30
 16  *
 17  *
 18  */
 19 
 20 
 21 #include<iostream>
 22 
 23 
 24 using namespace std;
 25 
 26 
 27 // 定义类
 28 class __data_structure{
 29 public:
 30         __data_structure(){ elem=0;}
 31         __data_structure(int t){ elem=t;}
 32         __data_structure(const __data_structure& ds){ elem = ds.elem; }
 33         void set_element(int t){ elem=t;}
 34         int get_element(){ return elem;}
 35         void msg(){ cout << "element = " << elem << endl;}
 36 
 37 private:
 38         int elem;
 39 };
 40 
 41 
 42 // 定义数据类型
 43 // typedef __data_structure* pointer_data_structure, data_structure;
 44 typedef __data_structure data_structure, *pointer_ds;
 45 
 46 
 47 template<typename T>
 48 T return_back(T t)
 49 {
 50         return t;
 51 }
 52 
 53 
 54 int main(int argc, char* argv[], char* envp[])
 55 {
 56 
 57         data_structure keep_return_value;
 58         pointer_ds pt_ds1=nullptr, pt_ds2=nullptr, keep_return_pointer=nullptr;
 59         data_structure  ds1(30);
 60         data_structure  ds2(60);
 61         pt_ds1 = &ds1;
 62         pt_ds2 = &ds2;
 63 
 64 
 65         //定义的类测试
 66 //      pt_ds1->msg();
 67 
 68 
 69         // parameter = value
 70         cout << "OS: keep_return_value " << endl;
 71         keep_return_value = return_back(ds2);
 72         keep_return_value.msg();
 73 
 74 
 75         // parameter = pointer
 76         cout << "OS: keep_return_pointer " << endl;
 77         keep_return_pointer = return_back(pt_ds2);
 78         keep_return_pointer -> msg();
 79 
 80 
 81 /*      说明：
 82  *
 83  *              1、return_back()函数返回的类型是T。
 84  *              2、当函数return_back()的参数类型是__data_structure， 返回值的类型是__data_structure；
 85  *              3、当函数return_back()的参数类型是__data_structure*， 返回值的类型是__data_structure*；
 86  *
 87  *      结论：因为“__data_structure”和“__data_structure*”都是使用“类型的参数T”返回的值，
 88  *              所以“ __data_structure* ”被视为“数据类型”。
 89  *
 90  *      疑问：拷贝构造函数（__data_structure(const __data_structure& ds) ）为什么可以直接调用private权限的变量elem（ds.elem ）?
 91  *
 92  */
 93 
 94 
 95         return 0;
 96 }
 97 [root@rockylinux tmp]#
 98 [root@rockylinux tmp]# 
 99 [root@rockylinux tmp]# g++ -o data_pointer  data_pointer.cpp 
100 [root@rockylinux tmp]# 
101 [root@rockylinux tmp]# 
102 [root@rockylinux tmp]# ./data_pointer
103 OS: keep_return_value
104 element = 60
105 OS: keep_return_pointer
106 element = 60
107 [root@rockylinux tmp]#
108 [root@rockylinux tmp]#

二、源码和运行结果：

  1 [root@rockylinux tmp]# uname -a
  2 Linux rockylinux 4.18.0-372.19.1.el8_6.x86_64 #1 SMP Tue Aug 2 16:19:42 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
  3 [root@rockylinux tmp]# 
  4 [root@rockylinux tmp]# 
  5 [root@rockylinux tmp]# g++  --version
  6 g++ (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10)
  7 Copyright (C) 2018 Free Software Foundation, Inc.
  8 This is free software; see the source for copying conditions.  There is NO
  9 warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 10 
 11 [root@rockylinux tmp]# 
 12 [root@rockylinux tmp]# 
 13 [root@rockylinux tmp]# cat parameter_function.cpp 
 14 /*
 15  *      date: 2022-08-30
 16  *      info: testing a function parameter of a function.
 17  */
 18 
 19 #include<iostream>
 20 
 21 
 22 using namespace std;
 23 
 24 
 25 // define funciton1 to be called.
 26 void msg1()
 27 {
 28         cout << "CPP:  hello, world!" << endl;
 29 }
 30 
 31 
 32 // define funciton2 to be called.
 33 int msg2(int var)
 34 {
 35         return var;
 36 }
 37 
 38 
 39 // define funciton3 to be called.
 40 int* msg3(int* pt)
 41 {
 42         return pt;
 43 }
 44 
 45 
 46 // define calling function1.
 47 void cpp_fun1( void(*fun1)())
 48 {
 49         fun1();
 50         cout <<"CPP: cpp_fun1" << endl;
 51 }
 52 
 53 
 54 // define calling function2, template function. 
 55 template<typename T>
 56 T cpp_fun2( T pt, T (*fun2)(T))
 57 {
 58         fun2(pt);
 59         cout <<"CPP: cpp_fun2" << endl;
 60         return fun2(pt);
 61 }
 62 
 63 
 64 // testing part.
 65 int main(int argc, char* argv[], char* envp[])
 66 {
 67 
 68         int keep_var, var = 40;
 69         int* pt_cpp_fun2_parameter, *keep_pt;
 70         pt_cpp_fun2_parameter = &var;
 71 
 72 
 73         // call cpp_fun1
 74         cpp_fun1(msg1);
 75 
 76 
 77         // call cpp_fun2
 78         keep_var = cpp_fun2(var, msg2);
 79         cout <<"CPP: cpp_fun2_return_general_value = " << keep_var << endl;
 80 
 81 
 82         // call cpp_fun2 with pointer parameter;
 83         keep_pt = cpp_fun2(pt_cpp_fun2_parameter, msg3);
 84         cout <<"CPP: cpp_fun2_return_pt_value = " << *keep_pt << endl;
 85 
 86 
 87         return 0;
 88 }
 89 [root@rockylinux tmp]# 
 90 [root@rockylinux tmp]# 
 91 [root@rockylinux tmp]# g++  -o parameter_function parameter_function.cpp 
 92 [root@rockylinux tmp]# 
 93 [root@rockylinux tmp]# 
 94 [root@rockylinux tmp]# ./parameter_function 
 95 CPP:  hello, world!
 96 CPP: cpp_fun1
 97 CPP: cpp_fun2
 98 CPP: cpp_fun2_return_general_value = 40
 99 CPP: cpp_fun2
100 CPP: cpp_fun2_return_pt_value = 40
101 [root@rockylinux tmp]# 
102 [root@rockylinux tmp]#

posted on 2022-08-30 14:49 lnlidawei 阅读(88) 评论(0) 编辑收藏举报

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