c++ - 命名空间和类中的可变参数模板函数

Question

我在类中定义了三个可变参数模板函数作为静态方法。然后我决定将它们移动到命名空间。命名空间方法编译失败，而类解决方案编译并按预期工作。

这是基于 worker 类(Class)的解决方案:

#include <list>
#include <string>

class sample
{
public:
    template<typename T>
    static std::string encode(T t) {
        /* do something useful with t */
        return std::string("encoded value");
    }

    template<typename... Ts>
    static std::string encode(Ts... ts) {
        std::list<std::string> values;
        return encode(values, ts...);
    }

    template<typename T, typename... Ts>
    static std::string encode(std::list<std::string>& values, T t, Ts... ts) {
        values.push_back(encode(t));
        return encode(values, ts...);
    }
};

命名空间中有类似的定义:

#include <list>
#include <string>

namespace sample
{
    template<typename T>
    std::string encode(T t) {
        /* do something useful with t */
        return std::string("encoded value");
    }

    template<typename... Ts>
    std::string encode(Ts... ts) {
        std::list<std::string> values;
        return encode(values, ts...);
    }

    template<typename T, typename... Ts>
    std::string encode(std::list<std::string>& values, T t, Ts... ts) {
        values.push_back(encode(t));
        return encode(values, ts...);
    }
};

在这两种情况下，编码的使用方式如下:

std::string encoded = sample::encode(1, 2u, 3.0);

命名空间方法失败并出现以下错误(第一行缩短):

sample_namespace.hpp: In instantiation of ‘std::__cxx11::string sample::encode(Ts ...) [with Ts = {std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, ...
sample_namespace.hpp:18:22:   recursively required from ‘std::__cxx11::string sample::encode(Ts ...) [with Ts = {std::__cxx11::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, int, unsigned int, double}]’
sample_namespace.hpp:18:22:   required from ‘std::__cxx11::string sample::encode(Ts ...) [with Ts = {int, unsigned int, double}]’
sample_namespace.hpp:17:32: fatal error: template instantiation depth exceeds maximum of 900 (use -ftemplate-depth= to increase the maximum)
     std::list<std::string> values;
                            ^~~~~~
compilation terminated.

显式定义 encode(int, uint, double) 时会出现同样的错误:

template std::string sample::encode<int, uint, double>(int a, uint b, double c);

当给出单个参数时，首先使用模板encode(T t) 并编译代码。

为什么放置在命名空间中的模板会失败？

以上代码是(未)使用 gcc 6.4.0 x86_64 编译的。我还尝试在启用 C++14 和 C++17 的情况下编译它。
代码也无法使用 gcc 7.3、clang 6.0.0 和 icc 18 进行编译 - 通过 godbolt.org 检查.

Answer 1

只需要将encode 的可变template 形式移动到非可变template 形式之前。在处理非成员函数时，顺序比成员函数重要得多。 Godbolt接受以下内容:

#include <list>
#include <string>

namespace sample
{
    template<typename T>
    std::string encode(T t) {
        /* do something useful with t */
        return std::string("encoded value");
    }

    template<typename T, typename... Ts>
    std::string encode(std::list<std::string>& values, T t, Ts... ts) {
    values.push_back(encode(t));
        return encode(values, ts...);
    }

    template<typename... Ts>
    std::string encode(Ts... ts) {
        std::list<std::string> values;
        return encode(values, ts...);
    }
};

std::string do_it() {
    return sample::encode(1, 2u, 3.0);
}