c++ - 递归模板调用中无法调用模板化函数重载

标签 c++ function templates overloading

我遇到了为某些模板化类型执行适当的函数模板重载的问题。看看我遇到的情况所需的最小示例如下所示:

#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。而这个序列化又需要序列化 ​​B 的 vector A 中包含的元素。 A 的所有序列化函数跑，意思是byte_content的味道, serialize和deserialize使用适当的定义进行调用。但是，当程序递归到通用 std::vector 时定义这些方法来序列化 std::vector<struct B> A 的数据成员，它无法调用为 B 定义的方法而是调用基本原语的序列化函数(前三个定义在代码示例的顶部)。我不明白为什么 byte_content 的序列化方法( serialize 、 deserialize 、 B )由于已定义，因此在这种情况下不会被调用。

我怀疑我缺少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

更准确地说，关键问题是当 A 序列化时发生时，它实际上会调用下面的预期方法

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，当它序列化b_elems时，它首先调用泛型 std::vector序列化方法 T = struct B 

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发生这种情况。

编辑2

添加@Evg推荐的前向声明后，代码几乎按照我的预期工作。现在唯一的问题是byte_content特化B没有被调用。可以通过将上述特化定义替换为 B 来验证这一点。与

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，正确的 byte_content特化B没有被调用。

最佳答案

注意:此答案引用编辑之前的问题(无前向声明)。

内部serialize(const std::vector<T>& data...)您使用了不合格的名称 serialize 。编译器应该决定哪个 serialize打电话。它将考虑 1) 在定义点可见的函数和 2) 在实例化点可由 ADL 找到的函数。两次查找都无法找到 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);

从一开始。

关于c++ - 递归模板调用中无法调用模板化函数重载，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/56697699/

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