Lambda闭包中的模板

Question

有人可以告诉我为什么以下代码

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，是否有解决方法？

||=== 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)) ===|

非常抱歉，几乎90%的问题都涉及代码，但我相信我的理解有误。我试图在lambda闭包内部使用模板参数。这在概念上是否可行？

- tubberd

Answer 1

Lambda函数默认标记为const，具有operator()，这意味着捕获的closure_function是const。然而，duk_get_args试图获取对const参数的非const引用，这就是编译器抱怨的原因。

要么：

1. 将duk_get_args更改为采用const func。
2. 在lambda的定义中添加mutable，以便可以对closure_function进行非const引用。

此外，closure_function变量是不必要的。您可以捕获function_item变量并使用它。这不是您的问题的一部分，但我想指出这一点。

- Cornstalks

---

非常感谢。看起来那就是问题所在。我没有意识到Lambda会将变量作为const引用捕获。 - tubberd