The following code compiles with gcc 4.5.1 but not with VS2010 SP1:
#include <iostream>
#include <vector>
#include <map>
#include <utility>
#include <set>
#include <algorithm>
using namespace std;
class puzzle
{
vector<vector<int>> grid;
map<int,set<int>> groups;
public:
int member_function();
};
int puzzle::member_function()
{
int i;
for_each(groups.cbegin(),groups.cend(),[grid,&i](pair<int,set<int>> group){
i++;
cout<<i<<endl;
});
}
int main()
{
return 0;
}
This is the error:
error C3480: 'puzzle::grid': a lambda capture variable must be from an enclosing function scope
warning C4573: the usage of 'puzzle::grid' requires the compiler to capture 'this' but the current default capture mode does not allow it
So,
1> which compiler is right?
2> How can I use member variables inside a lambda in VS2010?
I believe VS2010 to be right this time, and I'd check if I had the standard handy, but currently I don't.
Now, it's exactly like the error message says: You can't capture stuff outside of the enclosing scope of the lambda. † grid
is not in the enclosing scope, but this
is (every access to grid
actually happens as this->grid
in member functions). For your usecase, capturing this
works, since you'll use it right away and you don't want to copy the grid
auto lambda = [this](){ std::cout << grid[0][0] << "\n"; }
If however, you want to store the grid and copy it for later access, where your puzzle
object might already be destroyed, you'll need to make an intermediate, local copy:
vector<vector<int> > tmp(grid);
auto lambda = [tmp](){}; // capture the local copy per copy
† I'm simplifying - Google for "reaching scope" or see §5.1.2 for all the gory details.
capture this
:
auto lambda = [this](){};
use a local reference to the member:
auto& tmp = grid;
auto lambda = [ tmp](){}; // capture grid by (a single) copy
auto lambda = [&tmp](){}; // capture grid by ref
C++14:
auto lambda = [ grid = grid](){}; // capture grid by copy
auto lambda = [&grid = grid](){}; // capture grid by ref
example: https://godbolt.org/g/dEKVGD
我相信,你需要抓住this
。
An alternate method that limits the scope of the lambda rather than giving it access to the whole this
is to pass in a local reference to the member variable, eg
auto& localGrid = grid;
int i;
for_each(groups.cbegin(),groups.cend(),[localGrid,&i](pair<int,set<int>> group){
i++;
cout<<i<<endl;
});
To use the member variables inside Lambda expression in VS2010 and so on, you need to capture this pointer in capture list.
Please refer below sample code:
class A
{
public:
std::map<string, TicketInfo> m_mapImanFileUIDTicket;
void Myfunction();
};
void A::Myfunction()
{
std::string Filename;
NameRef namedRefOutput;
auto lambdaGetReadTicket = [FileName, namedRefOutput, this]()
{
TicketInfo Info;
Info.fileName = FileName;
Info.ticket = namedRefOutput.ticket;
m_mapImanFileUIDTicket[FileName] = Info; //This member variable defined in class
}
}
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.