我真的很困惑在 C++11 中返回大数据。最有效的方法是什么?
这是我的相关功能:
void numericMethod1(vector<double>& solution,
const double input);
void numericMethod2(pair<vector<double>,vector<double>>& solution1,
vector<double>& solution2,
const double input1,
const double input2);
这是我使用它们的方式:
int main()
{
// apply numericMethod1
double input = 0;
vector<double> solution;
numericMethod1(solution, input);
// apply numericMethod2
double input1 = 1;
double input2 = 2;
pair<vector<double>,vector<double>> solution1;
vector<double> solution2;
numericMethod2(solution1, solution2, input1, input2);
return 0;
}
问题是, std::move() 在以下实现中没有用吗?
执行:
void numericMethod1(vector<double>& solution,
const double input)
{
vector<double> tmp_solution;
for (...)
{
// some operation about tmp_solution
// after that this vector become very large
}
solution = std::move(tmp_solution);
}
void numericMethod2(pair<vector<double>,vector<double>>& solution1,
vector<double>& solution2,
const double input1,
const double input2)
{
vector<double> tmp_solution1_1;
vector<double> tmp_solution1_2;
vector<double> tmp_solution2;
for (...)
{
// some operation about tmp_solution1_1, tmp_solution1_2 and tmp_solution2
// after that the three vector become very large
}
solution1.first = std::move(tmp_solution1_1);
solution1.second = std::move(tmp_solution1_2);
solution2 = std::move(tmp_solution2);
}
如果它们没用,我如何在不多次复制的情况下处理这些大的返回值?
免费更改 API!
更新
感谢 StackOverFlow 和这些答案,在深入研究相关问题后,我更了解这个问题。由于 RVO,我更改了 API,为了更清楚,我不再使用 std::pair。这是我的新代码:
struct SolutionType
{
vector<double> X;
vector<double> Y;
};
SolutionType newNumericMethod(const double input1,
const double input2);
int main()
{
// apply newNumericMethod
double input1 = 1;
double input2 = 2;
SolutionType solution = newNumericMethod(input1, input2);
return 0;
}
SolutionType newNumericMethod(const double input1,
const double input2);
{
SolutionType tmp_solution; // this will call the default constructor, right?
// since the name is too long, i make alias.
vector<double> &x = tmp_solution.X;
vector<double> &y = tmp_solution.Y;
for (...)
{
// some operation about x and y
// after that these two vectors become very large
}
return tmp_solution;
}
我怎么知道 RVO 发生了?或者我如何 确保 RVO 发生?
最佳答案
按值返回,依赖 RVO (return value optimization) 。
auto make_big_vector()
{
vector<huge_thing> v1;
// fill v1
// explicit move is not necessary here
return v1;
}
auto make_big_stuff_tuple()
{
vector<double> v0;
// fill v0
vector<huge_thing> v1;
// fill v1
// explicit move is necessary for make_tuple's arguments,
// as make_tuple uses perfect-forwarding:
// http://en.cppreference.com/w/cpp/utility/tuple/make_tuple
return std::make_tuple(std::move(v0), std::move(v1));
}
auto r0 = make_big_vector();
auto r1 = make_big_stuff_tuple();
我会将您的函数的 API 更改为简单地按值返回。
关于c++ - C++11如何高效返回大数据,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/37117815/