試圖將向量的內容傳遞給線程

Question

我有一個函數需要一個字符串向量，並且我有一系列向量中的那些字符串向量。

我想循環遍歷每個向量傳遞給一個新線程的向量。

for (vector<vector<string> >::iterator it = vecstringvec.begin() ; 
    it != vecstringvec.end(); ++it){ 
    threadvector.push_back(thread(func, *it)); 
} 

本質上上述，除了上述不起作用（編譯錯誤）。我相信我需要一個向量的std :: ref傳遞，但我不確定，或者如何使用迭代器來做到這一點。

已滿錯誤：

/usr/include/c++/4.8/functional: In instantiation of ‘struct std::_Bind_simple<void (*(std::vector<std::basic_string<char> >, std::reference_wrapper<std::vector<std::basic_string<char> > >))(std::vector<std::basic_string<char> >&, std::vector<std::basic_string<char> >&)>’: 
/usr/include/c++/4.8/thread:137:47: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(std::vector<std::basic_string<char> >&, std::vector<std::basic_string<char> >&); _Args = {std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >&, std::reference_wrapper<std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >}]’ 
prog.cpp:199:55: required from here 
/usr/include/c++/4.8/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<void (*(std::vector<std::basic_string<char> >, std::reference_wrapper<std::vector<std::basic_string<char> > >))(std::vector<std::basic_string<char> >&, std::vector<std::basic_string<char> >&)>’ 
     typedef typename result_of<_Callable(_Args...)>::type result_type; 
                  ^
/usr/include/c++/4.8/functional:1727:9: error: no type named ‘type’ in ‘class std::result_of<void (*(std::vector<std::basic_string<char> >, std::reference_wrapper<std::vector<std::basic_string<char> > >))(std::vector<std::basic_string<char> >&, std::vector<std::basic_string<char> >&)>’ 
     _M_invoke(_Index_tuple<_Indices...>) 
     ^
make: *** [prog.o] Error 1 

Answer 1

尋找在該錯誤消息，func()向量參數是一個傳遞由引用，而不是通過噪聲值。嘗試使用std::ref(*it)。

cppreference引用：

The arguments to the thread function are copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref).

實施例：

#include <vector> 
#include <thread> 
#include <iostream> 

void func(const std::vector<int>& value) 
{ 
    std::cout << "Value: " << value.size() << std::endl; 
} 

int main(int argc, char const *argv[]) 
{ 
    std::vector<std::vector<int>> values = 
     { { 1, 2 } , { 1, 2, 3 }, { 1, 2, 3, 4 } }; 

    std::vector<std::thread> threads; 
    for (auto& vect: values) 
    { 
     threads.emplace_back(func, std::cref(vect)); 
    } 

    for (auto& thread: threads) 
    { 
     thread.join(); 
    } 

    return 0; 
} 

（順便說一句診斷本來更容易使用一個完整的自包含的例子）。