c++ - Function template materialization. I have been debugging for 1 hour. I have tried all means and the standard answers are correct, but it cannot be compiled.

Question

What I am doing is Question 6 of Chapter 8 on C++ Primer Plus
It is required to make two function templates, one for int double array, and one template to be concreted using char*[]
int and double There is no problem. I have been working on the concrete one for a long time. I don’t know why it can’t be compiled.
Then I wrote it again using ordinary functions and found that it can run normally. Then I worked on it for a long time and still couldn’t work, so I went online to find the answer. , I found that the format I wrote in answer
is basically the same. It just can’t be compiled

Error message: main.cpp:43:23: Error: template identifier 'maxn<const char [] of 'const char maxn(const char, int)' >'Does not match any template declaration template<>const char maxn<const char[]>(const char* x[], int n)

Code:

#include <cstring>
#include <iostream>

using namespace std;

template<typename T>T maxn(T x, int n);
template<>const char* maxn<const char*[]>(const char* x[], int n);

/*
 * 模板函数针对不同类型数组 输出数组中最大的那个值
 */
int main(int argc, char** argv) {
    int ix[6] = {34, 12, 343, 1, 43, 31};
    double dx[4] = {1.34, 1231.2, 34.3, 44.23};
    const char* sx[5] = {"abcde", "fedcba", "abcdefg", "12345", "qwert"};
    cout << "ix[6] max: " << *maxn(ix, sizeof ix / sizeof ix[0]) << endl;
    cout << "dx[4] max: " << *maxn(dx, sizeof dx / sizeof dx[0]) << endl;
    cout << "sx[5] max_address: " << (int*) maxn(sx, 5) << endl;
    cout << "Press Enter To Exit..." << endl;
    cin.get();
    return 0;
}
//这个是我自己写的普通函数,能运行
//const char* maxn(const char* x[5],int n) {
//    const char* max_len = x[0];
//    for (int i = 0; i < n-1; i++) {
//        max_len = strlen(max_len) >= strlen(x[i + 1]) ? max_len : x[i + 1];
//    }
//    return max_len;
//}

template<>const char* maxn<const char*[]>(const char* x[], int n)
{
    const char* max_len = x[0];
    for (int i = 0; i < n - 1; i++) {
        max_len = strlen(max_len) >= strlen(x[i + 1]) ? max_len : x[i + 1];
    }
    return max_len;
}

template<typename T>
T maxn(T x, int n) {
    for (int i = 0; i < n - 1; i++) {
        x[i + 1] = x[i] > x[i + 1] ? x[i] : x[i + 1];
    }
    return x + n - 1;
}

Answer 1

Statement: template<> const char **maxn(const char **x, int n);

Call: cout << "sx[5] max_address: " << (int*)*maxn(sx, 5) << endl;

Definition:

template<>
const char **maxn(const char **x, int n)
{
    const char **max_len = x;
    for (int i = 0; i < n - 1; i++)
        max_len = strlen(*max_len) >= strlen(x[i + 1]) ? max_len : &x[i + 1];
    return max_len;
}

You made three mistakes:

• Incorrect specialization syntax

• The two T types are inconsistent during specialization

• Many pointers are not dereferenced

---

Idea:

Template declaration: template<typename T> T maxn(T x, int n);
调用：maxn(sx, 5)。其中变量sx的类型是const char *[5], that is, array type.

Since sx is an array type, the form of the template formal parameter is T，这里sx会被隐式转换成指针类型（array to pointer conversion）const char **，即T是const char **.

So the specialization should be
template<> const char **maxn(const char **x, int n);

PS: Intuitively, there should be a specialization of the reference version. But because T cannot deduce the reference type, no specialization of the reference version is called here. Of course, you can provide template parameters to call this specialization.

Answer 2

According to my understanding, the maxn function should return the "largest" element in an array of length n, so I think the function declaration should be written like this:

template<typename T>T maxn(T x[], int n);
template<>const char* maxn<const char*>(const char* x[], int n);

Follow this statement, change your implementation, and it will compile.
The following code is simply modified based on your original code and compiled in vs2017 for reference:

template<typename T>T maxn(const T x[], int n);
template<>const char* maxn<const char*>(const char* const x[], int n);

template<>const char* maxn<const char*>(const char* const x[], int n)
{
    const char* max_len = x[0];
    for (int i = 0; i < n - 1; i++) {
        max_len = strlen(max_len) >= strlen(x[i + 1]) ? max_len : x[i + 1];
    }
    return max_len;
}

template<typename T> T maxn(const T x[], int n) {
    for (int i = 0; i < n - 1; i++) {
        x[i + 1] = x[i] > x[i + 1] ? x[i] : x[i + 1];
    }
    return x[n - 1];
}

Answer 3

You wrote the wrong special version.

template<>const char* maxn<const char*[]>(const char* x[], int n);

---

Correct posture:

const char* maxn(const char* x[], int n);