如何修改此函数，以便如果函数的参数是特定结构，则返回具有较大整数的结构?

How can I modify this function so that if the parameters of the function are specific structs it returns the struct with the bigger integer?

本文关键字：结构 函数 返回 何修改 整数 参数 如果 修改      更新时间：2023-10-16

我必须修改这个max函数，如果它处理一个struct Student，它将返回最大等级。 代码如下：

#include<iostream>
#include<string>
#include<cstring>
using namespace std;
typedef struct Student {
char* name;
int grade;
}Student;
template<typename  T>
T Max(T var1, T var2) {
if (sizeof(T) == sizeof(Student))
return(var1.grade > var2.grade) ? var1 : var2;
return (var1 > var2) ? var1 : var2;
}
int main() {
int i = 39;
int j = 20;
cout << "Max(i,j)=" << Max(i, j)<<endl;
double f1 = 13.5;
double f2 = 20.7;
cout << "Max(f1,f2)=" << Max(f1, f2) << endl;
string s1 = "Hello";
string s2 = "World";
cout << "Max(s1,s2)="<<Max(s1,s2) << endl;
Student first_student;
Student second_student;
first_student.name = new char[30];
second_student.name = new char[30];
strcpy(first_student.name, "Popescu David");
strcpy(second_student.name,"Gigel Petrovici");
first_student.grade = 7;
second_student.grade = 6;
cout << "Max(student1,student2)=" << Max(first_student, second_student).grade << endl;
return 0;
}

但是我得到这些错误：

1>------ Build started: Project: OOP, Configuration: Debug Win32 ------
1>LastTODO.cpp
1>c:usersdragossourcereposoopooplasttodo.cpp(11): error C2228: left of '.grade' must have class/struct/union
1>c:usersdragossourcereposoopooplasttodo.cpp(11): note: type is 'T'
1>        with
1>        [
1>            T=int
1>        ]
1>c:usersdragossourcereposoopooplasttodo.cpp(22): note: see reference to function template instantiation 'T Max<int>(T,T)' being compiled
1>        with
1>        [
1>            T=int
1>        ]
1>c:usersdragossourcereposoopooplasttodo.cpp(11): error C2039: 'grade': is not a member of 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>'
1>d:visual studiovctoolsmsvc14.16.27023includexstring(4373): note: see declaration of 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>'
1>c:usersdragossourcereposoopooplasttodo.cpp(28): note: see reference to function template instantiation 'T Max<std::string>(T,T)' being compiled
1>        with
1>        [
1>            T=std::string
1>        ]
1>c:usersdragossourcereposoopooplasttodo.cpp(12): error C2676: binary '>': 'T' does not define this operator or a conversion to a type acceptable to the predefined operator
1>        with
1>        [
1>            T=Student
1>        ]
1>c:usersdragossourcereposoopooplasttodo.cpp(38): note: see reference to function template instantiation 'T Max<Student>(T,T)' being compiled
1>        with
1>        [
1>            T=Student
1>        ]
1>Done building project "OOP.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

从 C++17 开始，您可以使用if constexpr请求以下行

return(var1.grade > var2.grade) ? var1 : var2;

仅当TStudent时才编译：

template<typename  T>
T Max(T var1, T var2) {
if constexpr ( std::is_same_v<T, Student> )
return(var1.grade > var2.grade) ? var1 : var2;
else
return (var1 > var2) ? var1 : var2;
}

演示

另一种可能性是将Max的主要版本定义为：

template<typename  T>
T Max(T var1, T var2) {
return (var1 > var2) ? var1 : var2;
}

并为Student结构提供专用化：

template<>
Student Max<Student>(Student var1, Student var2) {
return(var1.grade > var2.grade) ? var1 : var2;
}
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