c++ - 模板化函数重载未能在递归模板调用中被调用

我遇到了某些模板类型要执行适当的函数模板重载的问题。下面显示了查看我正在遇到的内容所需的最小示例:

#include <cstdio>
#include <vector>

template<typename id_type>
struct B {
    id_type ID;
    std::vector<int> values;
};

template<typename id_type>
struct A {
    id_type ID;
    std::vector<struct B<id_type>> b_elems;
};

// forward declarations
namespace aSDG {
    namespace meshing {
        template<typename id_type> size_t byte_content(const struct B<id_type>& instance);
        template<typename id_type> size_t serialize(const struct B<id_type>& instance, unsigned char* buffer, size_t start_idx = 0);
        template<typename id_type> size_t deserialize(struct B<id_type>& instance, const unsigned char* buffer, size_t start_idx = 0);
        template<typename id_type> size_t byte_content(const struct A<id_type>& instance);
        template<typename id_type> size_t serialize(const struct A<id_type>& instance, unsigned char* buffer, size_t start_idx = 0);
        template<typename id_type> size_t deserialize(struct A<id_type>& instance, const unsigned char* buffer, size_t start_idx = 0);
    }
}

namespace aSDG {
    namespace meshing {

        // serialization for primitive types
        template<typename T> size_t byte_content(const T& data){
            return sizeof(T);
        }

        template<typename T> size_t serialize(const T& data, unsigned char* buffer, size_t start_idx = 0)
        {
            std::memcpy((void*)(buffer + start_idx), (void*)&data, sizeof(data));
            return start_idx + sizeof(data);
        }
        template<typename T> size_t deserialize(T& data, const unsigned char* buffer, size_t start_idx = 0)
        {
            std::memcpy((void*)&data, (void*)(buffer + start_idx), sizeof(data));
            return start_idx + sizeof(data);
        }

        // serialization for vector containers
        template<typename T> size_t byte_content(const std::vector<T>& data){

            // get number of bytes for the size variable
            size_t num_req_bytes = sizeof(size_t);

            // get the number of bytes for each element of the vector
            for(size_t i = 0; i < data.size(); ++i){
                num_req_bytes += byte_content(data[i]);
            }// end for i

            // return the total number of required bytes
            return num_req_bytes;
        }

        template<typename T> size_t serialize(const std::vector<T>& data, unsigned char* buffer, size_t start_idx = 0)
        {
            // add the number of elements in the data
            const size_t size_ = data.size();
            start_idx = serialize(size_, buffer, start_idx);

            // add the actual data elements
            for(size_t i = 0; i < size_; ++i){
                start_idx = serialize(data[i], buffer, start_idx);
            }// end for i

            // return the final index after adding all the data
            return start_idx;
        }

        template<typename T> size_t deserialize(std::vector<T>& data, const unsigned char* buffer, size_t start_idx = 0)
        {
            // get the number of elements in the array
            size_t size_ = 0;
            start_idx = deserialize(size_, buffer, start_idx);

            // resize the input array
            data.resize(size_);

            // fill the array with the data in the buffer
            for(size_t i = 0; i < size_; ++i){
                start_idx = deserialize(data[i], buffer, start_idx);
            }// end for i

            // return the number of bytes we are at in the array
            return start_idx;
        }

    } // end namespace meshing
} // end namespace aSDG

namespace aSDG {
    namespace meshing {

        // serialization for B
        template<typename id_type>
        size_t byte_content(const struct B<id_type>& instance) {
            return byte_content(instance.ID) + byte_content(instance.values);
        }

        template<typename id_type>
        size_t serialize(const struct B<id_type>& instance, unsigned char* buffer, size_t start_idx){
            start_idx = serialize(instance.ID, buffer, start_idx);
            return serialize(instance.values, buffer, start_idx);
        }

        template<typename id_type>
        size_t deserialize(struct B<id_type>& instance, const unsigned char* buffer, size_t start_idx){
            start_idx = deserialize(instance.ID, buffer, start_idx);
            return deserialize(instance.values, buffer, start_idx);
        }

        // serialization functions for A
        template<typename id_type>
        size_t byte_content(const struct A<id_type>& instance) {
            return byte_content(instance.ID) + byte_content(instance.b_elems);
        }

        template<typename id_type>
        size_t serialize(const struct A<id_type>& instance, unsigned char* buffer, size_t start_idx){
            start_idx = serialize(instance.ID, buffer, start_idx);
            return serialize(instance.b_elems, buffer, start_idx);
        }

        template<typename id_type>
        size_t deserialize(struct A<id_type>& instance, const unsigned char* buffer, size_t start_idx){
            start_idx = deserialize(instance.ID, buffer, start_idx);
            return deserialize(instance.b_elems, buffer, start_idx);
        }


    } // end namespace meshing
} // end namespace aSDG



int main(int argc, const char * argv[]) {

    struct A<size_t> a1, a2;
    a1.b_elems.emplace_back();
    a1.b_elems.emplace_back();
    a1.b_elems.emplace_back();
    a1.b_elems[0].ID = 5;
    a1.b_elems[0].values.push_back(1);

    // get the number of bytes to be serialized
    size_t num_req_bytes = aSDG::meshing::byte_content(a1);

    // allocate the buffer
    std::vector<unsigned char> buf( num_req_bytes );

    // serialize the data in a1
    size_t serial_bytes = aSDG::meshing::serialize(a1, &buf[0]);

    // deserialize data into a2
    size_t deserial_bytes= aSDG::meshing::deserialize(a2, &buf[0]);

    // check that the bytes match
    printf("in_bytes = %zu vs. out_bytes = %zu\n", serial_bytes, deserial_bytes );

    return 0;
}

在此示例中，我将序列化A类型的实例，而该序列化又需要对B中包含的A元素的 vector 进行序列化。 A的所有序列化函数都会运行，这意味着将使用适当的定义调用byte_content，serialize和deserialize的样式。但是，当程序递归到这些方法的通用std::vector定义以序列化std::vector<struct B>的A数据成员时，它无法调用为B定义的方法，而是为基本原语调用了序列化函数(在Java中定义了前三个)代码示例的顶部)。我看不到为什么在这种情况下未调用byte_content的序列化方法(serialize，deserialize，B)，因为它们已定义。

我怀疑我缺少一些有关如何选择函数模板重载的基本规则，但我确实不确定。任何见识将不胜感激。

编辑1

更准确地说，关键问题是当A序列化发生时，实际上它将在下面调用预期的方法

template<typename id_type>
size_t aSDG::meshing::serialize(const struct A<id_type>& instance, unsigned char* buffer, size_t start_idx = 0){
    start_idx = serialize(instance.ID, buffer, start_idx);
    return serialize(instance.b_elems, buffer, start_idx);
}

问题是，当要序列化b_elems时，它首先使用std::vector调用通用T = struct B序列化方法

template<typename T> size_t serialize(const std::vector<T>& data, unsigned char* buffer, size_t start_idx = 0)
{
    // add the number of elements in the data
    const size_t size_ = data.size();
    start_idx = serialize(size_, buffer, start_idx);

    // add the actual data elements
    for(size_t i = 0; i < size_; ++i){
        start_idx = serialize(data[i], buffer, start_idx);
    }// end for i

    // return the final index after adding all the data
    return start_idx;
}

但是然后去做serialize(data[i], buffer, start_idx)时，该函数不会调用

template<typename id_type>
size_t serialize(const struct B<id_type>& instance, unsigned char* buffer, size_t start_idx = 0){
    start_idx = serialize(instance.ID, buffer, start_idx);
    return serialize(instance.values, buffer, start_idx);
}

而是打电话

template<typename T> size_t serialize(const T& data, unsigned char* buffer, size_t start_idx = 0)
{
    std::memcpy((void*)(buffer + start_idx), (void*)&data, sizeof(data));
    return start_idx + sizeof(data);
}

我真的很困惑为什么会这样。

编辑2

添加@Evg推荐的前向声明后，该代码几乎可以按我期望的那样工作。现在唯一的问题是没有调用byte_content的B专门化名称。可以通过将上述B的特化定义替换为来验证这一点

template<typename id_type>
size_t byte_content(const struct B<id_type>& instance) {
    printf("B byte_content\n");
    return byte_content(instance.ID) + byte_content(instance.values);
}

template<typename id_type>
size_t serialize(const struct B<id_type>& instance, unsigned char* buffer, size_t start_idx){
    printf("B serialize\n");
    start_idx = serialize(instance.ID, buffer, start_idx);
    return serialize(instance.values, buffer, start_idx);
}

template<typename id_type>
size_t deserialize(struct B<id_type>& instance, const unsigned char* buffer, size_t start_idx){
    printf("B deserialize\n");
    start_idx = deserialize(instance.ID, buffer, start_idx);
    return deserialize(instance.values, buffer, start_idx);
}

并证明“B byte_content”消息从未显示。现在也许我只是累了，没有看到一些错误，但是我看不到为什么，即使在前向声明之后，也没有调用byte_content的正确B专门化名称。

最佳答案

注意:此答案指的是编辑之前的问题(无前向声明)。

在serialize(const std::vector<T>& data...)内部，您使用了不合格的名称serialize。编译器应确定要调用的serialize。它将考虑功能1)在定义点可见，2)可以在实例化时由ADL找到。两次查找都将找不到serialize(const B<id_type>&...)。

一种可能的解决方案是提出声明

template<typename id_type>
size_t byte_content(const B<id_type>&);

template<typename id_type>
size_t serialize(const B<id_type>&, unsigned char*, size_t = 0);

template<typename id_type>
size_t deserialize(B<id_type>&, const unsigned char*, size_t = 0);

一开始

关于c++ - 模板化函数重载未能在递归模板调用中被调用，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/56697699/