第十三號艦隊
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Experience on C++ concepts

Posted on Edited on In

雖然Concept還沒正式列入C++ Standard中，不過提早經歷過總是好事..
Concept像Interface，定義一個靜態Concept該有些什麼東西
雖然可以用Interface完成，不過有時候我們不需要Runtime Polymorphism
常見的做法是用template來做

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#include <iostream>
#include <string>
template <typename T>
void print(T obj)
{
        std::cout << obj.c_str() <<  ", length: " << obj.length() << "\n";
}
int main()
{
        print(std::string("123"));
        print(123);  // Compile error
}

這方法最大的問題就是error message很難看懂, 常常跟Template打交道就有感覺了

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a.cpp: In instantiation of ‘void print(T) [with T = int]’:
a.cpp:23:11:   required from here
a.cpp:18:19: error: request for member ‘c_str’ in ‘obj’, which is of non-class type ‘int’
  std::cout << obj.c_str() <<  ", length: " << obj.length() << "\n";
               ~~~~^~~~~
a.cpp:18:51: error: request for member ‘length’ in ‘obj’, which is of non-class type ‘int’
  std::cout << obj.c_str() <<  ", length: " << obj.length() << "\n";

如果用Concept表達的話會是這個樣子

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#include <stdio.h>
#include <string>
template <class T>
concept bool Printable() {
    return requires(const T& obj) {
            { obj.c_str() }-> const char *;
            { obj.length() } -> size_t;
    };
}
void print(Printable obj)
{
        printf("%d: %s\n", obj.length(), obj.c_str());
}
int main()
{
        print(std::string("123"));
        print(123);
}
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a.cpp: In function ‘int main()’:
a.cpp:17:11: error: cannot call function ‘void print(auto:1) [with auto:1 = int]’
  print(123);
           ^
a.cpp:10:6: note:   constraints not satisfied
 void print(Printable obj)
      ^~~~~
a.cpp:4:14: note: within ‘template<class T> concept bool Printable() [with T = int]’
 concept bool Printable() {
              ^~~~~~~~~
a.cpp:4:14: note:     with ‘const int& obj’
a.cpp:4:14: note: the required expression ‘obj.c_str()’ would be ill-formed
a.cpp:4:14: note: the required expression ‘obj.length()’ would be ill-formed

面的Error告訴我們123不如的類型是int, 而int不符合 Printable這個Concept的要求

Limitiation

Concept只是Template的一層Wrapper而已，所以這樣的程式碼也是能通過的

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emplate <class T>
concept bool Printable() {
    return requires(const T& obj) {
            { obj.c_str() }-> const char *;
            { obj.length() } -> size_t;
    };
}
void print(Printable obj)
{
        if (!obj.empty()) {
                std::cout << obj.c_str() <<  ", length: " << obj.length() << "\n";
        }
}

明明Printable中沒定義empty的觀念，不過程式還是編譯通過，這個時候只能多加留意了