在递归模板调用中无法调用模板函数重载

Question

我正在运行遇到一个问题，即为某些模板类型执行适当的函数模板重载。下面显示了我运行正在研究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;
}

在这个例子中，我将序列化 A 类型的一个实例，而这个序列化又需要序列化包含在 A 中的 B 个元素的向量。 A 运行的所有序列化函数，意味着它的 byte_content、serialize 和 deserialize 的风格都使用适当的定义调用。但是，当程序递归到这些方法的通用 std::vector 定义以序列化 A 的 std::vector<struct B> 数据成员时，它无法调用为 B 定义的方法，并且而是为基本原语（代码示例顶部定义的前三个）调用序列化函数。我不明白为什么在这种情况下没有调用 B 的序列化方法（byte_content、serialize、deserialize），因为它们已定义。

我怀疑我遗漏了一些关于如何选择函数模板重载的基本规则，但我真的不确定。任何见解将不胜感激。

编辑 1

更准确的说，关键问题是当A的序列化发生时，它实际上会调用下面预期的方法

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，当它去序列化 b_elems 时，它首先使用 T = struct B

调用通用 std::vector 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;
}

但是当它执行 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);
}

而是调用

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，代码几乎可以按我的预期运行。现在唯一的问题是没有调用 B 的 byte_content 专业化。可以通过将 B 的上述专业化定义替换为

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);
}

并见证 "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，但我不明白为什么即使在前向声明之后，也没有调用 B 的正确 byte_content 专业化。

Answer 1

注意：此答案指的是编辑前的问题（无前向声明）。

在 serialize(const std::vector<T>& data...) 中，您使用了不合格的名称 serialize。编译器应该决定调用哪个 serialize 。它将考虑功能 1) 在定义时可见的功能和 2) 在实例化时可由 ADL 找到的功能。两次查找都找不到 serialize(const B<id_type>&...).

一个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。

在递归模板调用中无法调用模板函数重载

Templated function overload is failing to be called in recursive template call

c++

templates

overloading

function