How do I create a function object which modifies its arguments?

Question

I'd like to replace SomeFunction and SetArg with something more generic from boost. It looks like something that can be done with bind in combination with lambda, but I don't know how.

This code is very simple but the reason I'd like to replace it is because I need one for 2 and 3 etc arguments.

template<class T>
struct SomeFunction
{
    T value;
    SomeFunction(T s)
        : value(s) {}

    void operator()(T& s)
    {
        s = value;
    }
};

template<class T>
SomeFunction<T> SetArg(T value)
{
    return SomeFunction<T>(value);
}

The requirements:

• I want a function which returns a function object.
• When I call this function object, the parameters are passed by reference.
• The function modifies the objects passed in by reference by setting them to pre-set values.
• In the code above the pre-set values are passed in by value in the ctor, but any other way is also fine.

The following code demonstrates the usage:

void main()
{
    std::string t;
    SetArg(std::string("hello"))(t);
    assert(t == "hello");
}

Some context:

I want to test the client code of class Foo. So I want to replace the implementation of func1 with my own, but in a flexible way.

struct Foo
{
    virtual void func1(std::string& s)
    {
    }
};

struct MockFoo : public Foo {
    MOCK_METHOD1(func1, void(std::string&));
};

void ExampleTestCase::example()
{
  MockFoo f;
  std::string s;

  EXPECT_CALL(f, func1(_))
      .WillOnce(Invoke(SetArg(std::string("hello"))));

  f.func1(s);

  CPPUNIT_ASSERT_EQUAL(std::string("hello"), s);
}

Invoke takes a function or function object. Inside the new implementation of func1 it calls the function object returned by SetArg and sets its argument to the string "hello".

Invoke is part of gmock/gtest but SetArg is not.

Answer 1

Here is what I came up with. The operator() of setter would probably require some tweaking as we are not really benefiting from the possible move semantics here, but I can't figure that out right now.

Also note that this makes heavy use of C++11 features which might not be available to you.

#include <string>
#include <iostream>
#include <tuple>

// set arbitrary values to 
template<typename... Args>
struct setter {
  // needed because we cannot use initializer lists as they require assignment
  setter(const std::tuple<Args&...>& t) : t(t) {}
  std::tuple<Args...> t;

  // again a template to trigger deduction again
  template<typename... Args2>
  void operator()(Args2&&... args) {
    t = std::make_tuple(args...);
  }
};

template<typename... Args>
setter<Args&...> create_setter(Args&... args) {
  return setter<Args&...>(std::tie(args...));
}

int main()
{
  int i = 0;
  long l = 1;
  std::string foo = "foo";

  auto s = create_setter(i, l, foo);

  s(23, 42, "bar");
  std::cout << i << std::endl;
  std::cout << l << std::endl;
  std::cout << foo << std::endl;

  return 0;
}