lambda闭包中的模板

Question

任何人都可以告诉我为什么下面的代码

namespace detail
{
    ...

    template<typename ... T_Args>
    void duk_get_args(duk_context*& context,  std::function<void(T_Args...)>& func)
    {

        duk_get_args_impl<T_Args ...>(context, func, std::index_sequence_for<T_Args ...>());
    }
}
template<typename ... T_Args>
duk_c_function duk_function(std::function<void(T_Args ...)> function_item)
{
    std::function<void(T_Args ...)> closure_function = function_item;
    duk_c_function function_return = [closure_function] (duk_context* ctx) -> duk_ret_t {
        detail::duk_get_args<T_Args ...>(ctx, closure_function);
        return 0;
    };
    return function_return;
}

抛出以下错误,如果有解决方法吗？

||=== Build: Release_win64 in dukbind (compiler: GNU GCC Compiler) ===|
include\dukbind\detail.hpp||In instantiation of 'dukbind::duk_function(std::function<void(T_Args ...)>)::<lambda(duk_context*)> [with T_Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}; duk_ret_t = int; duk_context = duk_hthread]':|
include\dukbind\detail.hpp|81|required from 'struct dukbind::duk_function(std::function<void(T_Args ...)>) [with T_Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}; dukbind::duk_c_function = std::function<int(duk_hthread*)>]::<lambda(duk_context*)>'|
include\dukbind\detail.hpp|85|required from 'dukbind::duk_c_function dukbind::duk_function(std::function<void(T_Args ...)>) [with T_Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}; dukbind::duk_c_function = std::function<int(duk_hthread*)>]'|
C:\Users\Daniel Wanner\Documents\Projekte\C++\dukbind\src\main.cpp|166|required from here|
include\dukbind\detail.hpp|83|error: no matching function for call to 'duk_get_args(duk_context*&, const std::function<void(std::__cxx11::basic_string<char>)>&)'|
include\dukbind\detail.hpp|70|note: candidate: template<class ... T_Args> void dukbind::detail::duk_get_args(duk_context*&, std::function<void(T_Args ...)>&)|
include\dukbind\detail.hpp|70|note:   template argument deduction/substitution failed:|
include\dukbind\detail.hpp|83|note:   types 'std::function<void(T_Args ...)>' and 'const std::function<void(std::__cxx11::basic_string<char>)>' have incompatible cv-qualifiers|
C:\TDM-GCC-64\lib\gcc\x86_64-w64-mingw32\5.1.0\include\c++\functional|2250|error: 'std::function<_Res(_ArgTypes ...)>::function(_Functor) [with _Functor = dukbind::duk_function(std::function<void(T_Args ...)>) [with T_Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}; dukbind::duk_c_function = std::function<int(duk_hthread*)>]::<lambda(duk_context*)>; <template-parameter-2-2> = void; _Res = int; _ArgTypes = {duk_hthread*}]', declared using local type 'dukbind::duk_function(std::function<void(T_Args ...)>) [with T_Args = {std::__cxx11::basic_|
||=== Build failed: 2 error(s), 4 warning(s) (0 minute(s), 0 second(s)) ===|

对于近90%的代码问题,我很抱歉,但我相信我的概念有些不对劲.我试图在lambda闭包内使用模板参数.甚至在概念上可能吗？

Answer 1

Lambda函数有一个默认operator()标记const的,这意味着捕获的closure_function是const.但是,duk_get_args尝试对参数进行非const引用const,这就是编译器所抱怨的.

或者:

1. 改变duk_get_args采取const func.
2. 添加mutable到lambda的定义,以便您可以不const参考closure_function.

另外,closure_function变量是不必要的.您可以捕获function_item变量并使用它.这不是你问题的一部分,但我想指出的是.