Anonymous objects in C++

There are not many cases in which you can have an anonymous *anything* in your code, yet there’s an idiom in C++ which lets you use an object with an anonymous type. Like this:

void foo()
{
   struct {
      int x;
   };
}

Why would this be useful, you may ask. That’s a valid question. This idiom can be very useful to write callbacks, like this:

struct Interface {
   void callback() = 0;
};

void bar(Interface &c);

void foo()
{
   struct : public Interface {
      /* ... */
   } x;

   bar(x);
}

I am not aware of many other uses for an anonymous type. Even more considering this idiom can now be replaced with a much cleaner lambda. But hey, it looks cool!

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Fixing end of line styles between Linux and Windows with SVN

Quite a mouthful for such an easy thing. Don’t you just hate when half the people in a project use CR/LF and the other half just LF?

Luckly this is easy to fix, assuming you are using svn. You can use something called auto-props to setup the eol style for different file types.

Set it once for the project, never worry again. Anyone knows its git counterpart?


C++: Checking if a method exists in a parent class

I like Google test and Google mock (C++ only) a lot. These are really great tools to ensure the quality of your code. They do have one problem however, especially when working with a legacy codebase: many times you need to change a signature for a function and your tests begin to fail. Those tests shouldn’t really fail, they shouldn’t compile at all because you’re now trying to mock a function which doesn’t exists anymore.

I worked on a patch to check if a class and its parent share the same methods, but I hit some major roadblocks which I believe cannot be saved:

  • You have no way of detecting if the parent’s method is actually virtual. It may have the same signature, yet if it isn’t virtual the mock serves no purpose
  • You have no (easy) way of detecting if the method is defined in your parent’s parent

Even though this code will never be useful, I thought I might as well post this here, just in case anyone comes up with a solution for those problems.

// :!g++ foo.cpp -lgtest_main -lgmock && ./a.out
#include <gtest/gtest.h>
#include <gmock/gmock.h>
using namespace testing;

/**
 * "Real" application
 */
class Foo {
	public:
	virtual int bar(int) { return 1; }
	//virtual int bar(void) { return 1; }
};

class Do {
	public:
	int something(Foo &foo){ return foo.bar(1); }
};

/**
 * Class used to compare method ptrs. We need to inherit from class C
 * to forward the calls to the derived methods.
 */
template <class C, class D>
class Mocks_Must_Exist_In : public C {
	public:
	// We don't know in the derived class the typeof the parent class
	// so we define a common name using some template magic
	typedef C ParentClass;
	// Actually we don't know our own class either
	typedef D Self;

	// Function ptr definitions are ugly so we might as well use
	// a template to hide it under the rug
	template <class F, class G>
	void mock_created_for_unexisting_method(F f, G g){ f = g; }
};

#define METHOD_EXISTS(Method) 
	void defined_##Method() { 
		mock_created_for_unexisting_method(&Self::Method, &ParentClass::Method); 
	} 

/**
 * Checked mock, shouldn't compile if Foo's interface changes
 */
class FooMock : public Mocks_Must_Exist_In< Foo, FooMock > {
	public:
	MOCK_METHOD1(bar, int(int));
	METHOD_EXISTS(bar);
};

/**
 * Unchecked mock, should compile if Foo's interface changes
 */
class FooMock2 {
	public:
	MOCK_METHOD0(bar, int());
};

TEST(FooTest, ThisShouldCompile) {
	FooMock foo;
	EXPECT_CALL(foo, bar(_)).WillOnce(Return(42));
	EXPECT_EQ(42, Do().something(foo));
}

Vim Tip: Converting to a Beamer frame

Today we’ll learn how to convert the text under the cursor to a Beamer frame, yet it’s easy to apply the same technique for other stuff:

map xibegin{frame}{TITLE}end{frame}kp

Lets analyze it
* xi cut whatever is selected and enter insert mode
* begin{frame}{TITLE}end{frame} write the beamer frame declaration
* kp go up one line and paste what was on the buffer

You can do something like this whenever you need to map text in Vim, it’s very easy.