How to get rid of temp and use one transform to get job done?

Question

 vector<double> result(vectorOfClassA.size());
 vector<classB> temp(vectorOfClassA.size());
 std::transform(vectorOfClassA.begin(), vectorOfClassA.end(), back_inserter(temp),
                std::tr1::bind(&A::memberVariableOfClassB, std::tr1::placeholders::_1));
 std::transform(temp.begin(), temp.end(), back_inserter(result),
                 std::tr1::bind(&B::getValue, std::tr1::placeholders::_1));

I like to use one transform but it can't compile

std::transform(vectorOfClassA.begin(), vectorOfClassA.end(), back_inserter(result),
               std::tr1::bind(&B::getValue,
                              std::tr1::bind(&A::memberVariableOfClassB,
                                             std::tr1::placeholders::_1)));

Here double getValue() is class B member function. How to do it or what is wrong with my code?

Could the reason for the failed compile simply be that you misspelled `memberVariableOfClassB`? — Rob Kennedy, Oct 19 '12 at 14:24
Then how do you explain that your "failed" code says `memberVariableOfCallB`? *Call* is not the same as *Class*. — Rob Kennedy, Oct 19 '12 at 15:29
memberVariableOfCallB here should be type of classB member Variable, not classB, am I right? — user1759558, Oct 19 '12 at 16:09
Your first code block says `memberVariableOfClassB`, but the second code block says `memberVariableOfCallB`. Do you see the difference? Do you understand why that difference is important? I can't tell you what it's supposed to be since I can't see the rest of your code. It's *your* code, so you should already know what the name of your own variable or function is. — Rob Kennedy, Oct 19 '12 at 17:51
It is my fault. It is memberVariableOfClassB. My code use concrete class name. Thanks anyway. — user1759558, Oct 19 '12 at 17:58

score 2 · Answer 1 · answered Oct 19 '12 at 14:20

2

If your compiler supports C++11, you can make use of a lambda:

std::transform(vectorOfClassA.begin(), vectorOfClassA.end(), 
  back_inserter(result),
  [] (const A& a)
  {
  return a.memberVariableOfClassB.GetValue();
  });

answered Oct 19 '12 at 14:20

Andriy

8,486
3
27
51

2

If your compiler is old, you could use a regular function instead of a lambda function (same idea) – Kos Oct 19 '12 at 14:21
@user1759558: It is a capture clause of a lambda expression. [What is a lambda expression in C++11?](http://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11) – Andriy Oct 19 '12 at 14:40

How to get rid of temp and use one transform to get job done?

1 Answers1