C ++ 11 emplace_back on vector< struct&gt ;? [英] C++11 emplace_back on vector<struct>?

问题描述

请考虑以下程序：

  #include< string& 
 #include< vector> 
 
 using namespace std; 
 
 struct T 
 {
 int a; 
 double b; 
 string c; 
}; 
 
矢量< T> V; 
 
 int main（）
 {
 V.emplace_back（42，3.14，foo）; 
} 
  

不起作用：

  $ g ++ -std = gnu ++ 11 ./test.cpp 
在从/usr/include/c++/4.7/x86_64-linux-gnu /bits/c++allocator.h:34:0，
来自/usr/include/c++/4.7/bits/allocator.h:48，
来自/usr/include/c++/4.7/ string：43，
 from ./test.cpp:1：
 /usr/include/c++/4.7/ext/new_allocator.h：在'void __gnu_cxx :: new_allocator< _Tp> ::构造（_Up *，_Args&& ...）[with _Up = T; _Args = {int，double，const char（&）[4]}; _Tp = T]'：
 /usr/include/c++/4.7/bits/alloc_traits.h:253:4：需要从'static typename std :: enable_if< std :: allocator_traits< _Alloc> :: __ construct_helper& _Tp，_Args> :: value，void> :: type std :: allocator_traits< _Alloc> :: _ S_construct（_Alloc& _Tp *，_Args& _Args = {int，double，const char（&）[4]}; _Alloc = std :: allocator< T> ;; typename std :: enable_if< std :: allocator_traits< _Alloc> :: __ construct_helper< _Tp，_Args> :: value，void> :: type = void]'
 /usr/include/c++/4.7/bits/alloc_traits .h：390：4：需要从'static void std :: allocator_traits< _Alloc> :: construct（_Alloc& _Tp *，_Args&& ...）[with _Tp = T; _Args = {int，double，const char（&）[4]}; _Alloc = std :: allocator< T>]'
 /usr/include/c++/4.7/bits/vector.tcc:97:6：需要从'void std :: vector< _Tp，_Alloc> :: emplace_back （_Args& ...）[with _Args = {int，double，const char（&）[4]}; _Tp = T; _Alloc = std :: allocator< T>]'
 ./test.cpp:17:32：需要从这里
 /usr/include/c++/4.7/ext/new_allocator.h:110:4 ：error：没有匹配函数调用'T :: T（int，double，const char [4]）'
 /usr/include/c++/4.7/ext/new_allocator.h:110:4：note ：candidate are：
 ./test.cpp:6:8：note：T :: T（）
 ./test.cpp:6:8：note：candidate expects 0 arguments，3 provided 
 ./test.cpp:6:8：note：T :: T（const T&）
 ./test.cpp:6:8：note：candidate expects 1 argument，3提供
 ./test.cpp:6:8：note：T :: T（T&&）
 ./test.cpp:6:8：note：candidate expects 1 argument，3提供
  

这样做的正确方法是什么，为什么？

$ c>语法以初始化新元素：

  V.emplace_back（T {42，3.14，foo}）; 
  

Consider the following program:

#include <string>
#include <vector>

using namespace std;

struct T
{
    int a;
    double b;
    string c;
};

vector<T> V;

int main()
{
    V.emplace_back(42, 3.14, "foo");
}

It doesn't work:

$ g++ -std=gnu++11 ./test.cpp
In file included from /usr/include/c++/4.7/x86_64-linux-gnu/bits/c++allocator.h:34:0,
                 from /usr/include/c++/4.7/bits/allocator.h:48,
                 from /usr/include/c++/4.7/string:43,
                 from ./test.cpp:1:
/usr/include/c++/4.7/ext/new_allocator.h: In instantiation of ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = T; _Args = {int, double, const char (&)[4]}; _Tp = T]’:
/usr/include/c++/4.7/bits/alloc_traits.h:253:4:   required from ‘static typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type std::allocator_traits<_Alloc>::_S_construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = T; _Args = {int, double, const char (&)[4]}; _Alloc = std::allocator<T>; typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type = void]’
/usr/include/c++/4.7/bits/alloc_traits.h:390:4:   required from ‘static void std::allocator_traits<_Alloc>::construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = T; _Args = {int, double, const char (&)[4]}; _Alloc = std::allocator<T>]’
/usr/include/c++/4.7/bits/vector.tcc:97:6:   required from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {int, double, const char (&)[4]}; _Tp = T; _Alloc = std::allocator<T>]’
./test.cpp:17:32:   required from here
/usr/include/c++/4.7/ext/new_allocator.h:110:4: error: no matching function for call to ‘T::T(int, double, const char [4])’
/usr/include/c++/4.7/ext/new_allocator.h:110:4: note: candidates are:
./test.cpp:6:8: note: T::T()
./test.cpp:6:8: note:   candidate expects 0 arguments, 3 provided
./test.cpp:6:8: note: T::T(const T&)
./test.cpp:6:8: note:   candidate expects 1 argument, 3 provided
./test.cpp:6:8: note: T::T(T&&)
./test.cpp:6:8: note:   candidate expects 1 argument, 3 provided

What is the correct way to do this and why?

(Also tried single and double braces)

解决方案

You can use the {} syntax to initialize the new element:

V.emplace_back(T{42, 3.14, "foo"});

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