initialize user-defined objects using lamda expression

[sajis997]

Hello forum,

I have declared smart pointer as follows:

        std::vector<std::shared_ptr<Line>> lines_;
        std::vector<std::shared_ptr<Line>> edgeLines_;

Now I want to call make_shared<Line> over each element of the vector I have declared above. Would lamda expression be a better option than function objects ?

I am not sure how to formulate the syntax.

Thanks

[sajis997]

I tried the following : Please let me know how to validate it and is there any better way to accomplish this:

#include <string>
#include <iostream>
#include <algorithm>
#include <vector>
#include <memory>

using namespace std;


class A
{
public:

  A():val(nullptr) {}
  ~A() { std::cout << "Destructor called" << std::endl;}

private:
  int *val;
};


int main()
{

  std::vector<std::shared_ptr<A>> allAs;

  allAs.resize(120);

  for_each(allAs.begin(),allAs.end(),[](std::shared_ptr<A>) -> std::shared_ptr<A>  {return std::make_shared<A>();});


  return 0;

}

[Peter87]

The code looks like it should work, but why are you not using a regular for loop (or a range-based for loop)? I fail to see what advantage std::for_each has in this situation.

[sajis997]

I think I am doing something wrong. I tried to check if the memory is actually created with the following snippet and it prints that no memory is allocated.

#include <string>
#include <iostream>
#include <algorithm>
#include <vector>
#include <memory>

using namespace std;


class A
{
public:

  A():val(nullptr) { /*std::cout << "Constructor called" << std::endl; */}
  ~A() { /*std::cout << "Destructor called" << std::endl; */}

private:
  int *val;
};


int main()
{

  std::vector<std::shared_ptr<A>> allAs;

  allAs.resize(120);

  std::for_each(allAs.begin(),allAs.end(),[](std::shared_ptr<A> as)  mutable {as = std::make_shared<A>();});

  //check that memory is actually created
  
  int notInitialized = std::count_if(allAs.cbegin(),allAs.cend(),
				    [](std::shared_ptr<A> as)
				     { return as.get() == 0;});

  std::cout << "Number of un-initialized memory: " << notInitialized << std::endl;
  
  return 0;

}

Is the parameter is passed by value ? In this case even copy be value should show the expected result.


Any idea ?

[JLBorges]

Pass by reference: we want to modify the object in the vector.

// std::for_each(allAs.begin(),allAs.end(),[](std::shared_ptr<A> as)  mutable {as = std::make_shared<A>();});
std::for_each( allAs.begin(), allAs.end(),
               []( std::shared_ptr<A>& ptr )  { ptr = std::make_shared<A>(); } );

Or, simpler: for( auto& ptr : allAs ) ptr = std::make_shared<A>();