假设我有两个标准向量：
vector<int> a;
vector<int> b;
我们也可以说两者都有大约30个元素。
如何将向量b添加到向量a的末尾？
肮脏的方法是遍历b并通过vector<int>：：push_back（）添加每个元素，尽管我不想这样做！
当前回答
如果您想将vector添加到自身，两种流行的解决方案都将失败：
std::vector<std::string> v, orig;
orig.push_back("first");
orig.push_back("second");
// BAD:
v = orig;
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "", "" }
// BAD:
v = orig;
std::copy(v.begin(), v.end(), std::back_inserter(v));
// std::bad_alloc exception is generated
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "first", "second" }
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
std::copy(v.begin(), v.end(), std::back_inserter(v));
// Now v contains: { "first", "second", "first", "second" }
// GOOD (best):
v = orig;
v.insert(v.end(), orig.begin(), orig.end()); // note: we use different vectors here
// Now v contains: { "first", "second", "first", "second" }
其他回答
a.insert(a.end(), b.begin(), b.end());
or
a.insert(std::end(a), std::begin(b), std::end(b));
第二种变体是更通用的解决方案，因为b也可以是数组。然而，它需要C++11。如果要使用用户定义的类型，请使用ADL：
using std::begin, std::end;
a.insert(end(a), begin(b), end(b));
std::copy (b.begin(), b.end(), std::back_inserter(a));
这可以用于向量a中的项没有赋值运算符（例如常量成员）的情况。
在所有其他情况下，与上述插入解决方案相比，该解决方案是无效的。
虽然说“编译器可以保留”，但为什么要依赖它呢？那么移动语义的自动检测呢？那个么，那个些以开头和结尾重复容器名称的做法呢？
你不想要更简单的东西吗？
（向下滚动到主菜单，以查看要点）
#include <type_traits>
#include <vector>
#include <iterator>
#include <iostream>
template<typename C,typename=void> struct can_reserve: std::false_type {};
template<typename T, typename A>
struct can_reserve<std::vector<T,A>,void>:
std::true_type
{};
template<int n> struct secret_enum { enum class type {}; };
template<int n>
using SecretEnum = typename secret_enum<n>::type;
template<bool b, int override_num=1>
using EnableFuncIf = typename std::enable_if< b, SecretEnum<override_num> >::type;
template<bool b, int override_num=1>
using DisableFuncIf = EnableFuncIf< !b, -override_num >;
template<typename C, EnableFuncIf< can_reserve<C>::value >... >
void try_reserve( C& c, std::size_t n ) {
c.reserve(n);
}
template<typename C, DisableFuncIf< can_reserve<C>::value >... >
void try_reserve( C& c, std::size_t ) { } // do nothing
template<typename C,typename=void>
struct has_size_method:std::false_type {};
template<typename C>
struct has_size_method<C, typename std::enable_if<std::is_same<
decltype( std::declval<C>().size() ),
decltype( std::declval<C>().size() )
>::value>::type>:std::true_type {};
namespace adl_aux {
using std::begin; using std::end;
template<typename C>
auto adl_begin(C&&c)->decltype( begin(std::forward<C>(c)) );
template<typename C>
auto adl_end(C&&c)->decltype( end(std::forward<C>(c)) );
}
template<typename C>
struct iterable_traits {
typedef decltype( adl_aux::adl_begin(std::declval<C&>()) ) iterator;
typedef decltype( adl_aux::adl_begin(std::declval<C const&>()) ) const_iterator;
};
template<typename C> using Iterator = typename iterable_traits<C>::iterator;
template<typename C> using ConstIterator = typename iterable_traits<C>::const_iterator;
template<typename I> using IteratorCategory = typename std::iterator_traits<I>::iterator_category;
template<typename C, EnableFuncIf< has_size_method<C>::value, 1>... >
std::size_t size_at_least( C&& c ) {
return c.size();
}
template<typename C, EnableFuncIf< !has_size_method<C>::value &&
std::is_base_of< std::random_access_iterator_tag, IteratorCategory<Iterator<C>> >::value, 2>... >
std::size_t size_at_least( C&& c ) {
using std::begin; using std::end;
return end(c)-begin(c);
};
template<typename C, EnableFuncIf< !has_size_method<C>::value &&
!std::is_base_of< std::random_access_iterator_tag, IteratorCategory<Iterator<C>> >::value, 3>... >
std::size_t size_at_least( C&& c ) {
return 0;
};
template < typename It >
auto try_make_move_iterator(It i, std::true_type)
-> decltype(make_move_iterator(i))
{
return make_move_iterator(i);
}
template < typename It >
It try_make_move_iterator(It i, ...)
{
return i;
}
#include <iostream>
template<typename C1, typename C2>
C1&& append_containers( C1&& c1, C2&& c2 )
{
using std::begin; using std::end;
try_reserve( c1, size_at_least(c1) + size_at_least(c2) );
using is_rvref = std::is_rvalue_reference<C2&&>;
c1.insert( end(c1),
try_make_move_iterator(begin(c2), is_rvref{}),
try_make_move_iterator(end(c2), is_rvref{}) );
return std::forward<C1>(c1);
}
struct append_infix_op {} append;
template<typename LHS>
struct append_on_right_op {
LHS lhs;
template<typename RHS>
LHS&& operator=( RHS&& rhs ) {
return append_containers( std::forward<LHS>(lhs), std::forward<RHS>(rhs) );
}
};
template<typename LHS>
append_on_right_op<LHS> operator+( LHS&& lhs, append_infix_op ) {
return { std::forward<LHS>(lhs) };
}
template<typename LHS,typename RHS>
typename std::remove_reference<LHS>::type operator+( append_on_right_op<LHS>&& lhs, RHS&& rhs ) {
typename std::decay<LHS>::type retval = std::forward<LHS>(lhs.lhs);
return append_containers( std::move(retval), std::forward<RHS>(rhs) );
}
template<typename C>
void print_container( C&& c ) {
for( auto&& x:c )
std::cout << x << ",";
std::cout << "\n";
};
int main() {
std::vector<int> a = {0,1,2};
std::vector<int> b = {3,4,5};
print_container(a);
print_container(b);
a +append= b;
const int arr[] = {6,7,8};
a +append= arr;
print_container(a);
print_container(b);
std::vector<double> d = ( std::vector<double>{-3.14, -2, -1} +append= a );
print_container(d);
std::vector<double> c = std::move(d) +append+ a;
print_container(c);
print_container(d);
std::vector<double> e = c +append+ std::move(a);
print_container(e);
print_container(a);
}
呵呵。
现在，有了@DyP的帮助，从rhs移动数据，将数组追加到容器，将forward_list追加到容器中，将容器从lhs移动。
请注意，由于EnableFunctionIf＜＞。。。技巧在铿锵声中，这种变通方法奏效了。
如果您想将vector添加到自身，两种流行的解决方案都将失败：
std::vector<std::string> v, orig;
orig.push_back("first");
orig.push_back("second");
// BAD:
v = orig;
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "", "" }
// BAD:
v = orig;
std::copy(v.begin(), v.end(), std::back_inserter(v));
// std::bad_alloc exception is generated
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
v.insert(v.end(), v.begin(), v.end());
// Now v contains: { "first", "second", "first", "second" }
// GOOD, but I can't guarantee it will work with any STL:
v = orig;
v.reserve(v.size()*2);
std::copy(v.begin(), v.end(), std::back_inserter(v));
// Now v contains: { "first", "second", "first", "second" }
// GOOD (best):
v = orig;
v.insert(v.end(), orig.begin(), orig.end()); // note: we use different vectors here
// Now v contains: { "first", "second", "first", "second" }
