STL SGI 2.9.2版本源码中vector的实现细节。

insert(iterator position, size_type n, const T& x)

/*
 * 从position处（包括）开始插入n个相同的元素x
 * */
template<class T, class Alloc>
void vector<T, Alloc>::insert(iterator position, size_type n, const T &x) {
    if (n != 0) {
        // end_of_storage表示可用空间的尾, finish表示未使用空间内存的起始处
        // 可用空间个数大于等于n
        if (size_type(end_of_storage - finish) >= n) {
            T x_copy = x;
            const size_type elems_after = finish - position;
            iterator old_finish = finish;

            // 插入点之后的现有元素个数大于新增元素个数
            if (elems_after > n) {
                // uninitialized_copy将迭代器first和last指定的范围内中的元素拷贝到以迭代器result起始的区域内
                // 从finish - n 到finish(不包括finish)共有n个元素
                uninitialized_copy(finish - n, finish, finish);
                finish += n;

                // 把[position, old_finish - n)中的元素拷贝到[old_finish - n - position, old_finish)指定的范围内
                copy_backward(position, old_finish - n, old_finish);
                fill(position, position + n, x_copy);
            } else {
                // 插入点之后的现有元素个数小于新增元素个数
                // 为以finish开始的内存空间指定n-elems_after个x_copy的初值
                // 返回值：被赋值的尾元素下一位置的迭代器
                uninitialized_fill_n(finish, n - elems_after, x_copy);
                finish += n - elems_after;
                uninitialized_copy(position, old_finish, finish);
                finish += elems_after;
                fill(position, old_finish, x_copy);
            }
        } else {
            const size_type old_size = size();
            const size_type len = old_size + max(old_size, n);
            iterator new_start = data_allocator::allocate(len);

            // 配置新的内存空间
            iterator new_finish = new_start;
            __STL_TRY{
                    // 将插入点之前的元素拷贝到新空间中
                    new_finish = uninitialized_copy(start, position, new_start);

                    // 插入新元素
                    new_finish = uninitialized_fill_n(new_finish, n, x);

                    // 将插入点之后的元素拷贝到新空间中
                    new_finish = uninitialized_copy(position, finish, new_finish);
            }

            // 如有异常发生，实现"commit or rollback" semantics
#         ifdef  __STL_USE_EXCEPTIONS
            catch(...) {
              destroy(new_start, new_finish);
              data_allocator::deallocate(new_start, len);
              throw;
            }
#         endif /* __STL_USE_EXCEPTIONS */
            // 调用旧的元素的额析构函数,释放旧的内存空间
            destroy(start, finish);
            deallocate();

            // 调整迭代器
            start = new_start;
            finish = new_finish;
            end_of_storage = new_start + len;
        }
    }
}