I'm converting a C++ project to C

The major issues are as follows:

structs/classes -
classes with functions not allowed in C so I have to pull them out and rename them/change parameters
no constructors or destructors
so I need init and destroy methods

new/delete -
all of these have to be replaced with malloc/free with corresponding init and destroy calls.

the use of booleans has to be corrected since there is none in C, but this is fixable with a simple typedef and find/replace.

the biggest problem is templates.

how the hell do I get rid of template data structures/classes elegantly. almost every solution that I can think of is beyond sloppy

If you have code that relies heavily on templates, then you are probably going to end up with a lot of hard work. Simple templates can sometimes be replaced by #define macros, but it's tricky and far from trivial. Some ways of using templates is almost impossible to replace.

What is the reason you feel the need to translate to C anyways - don't tell me speed - if that's your reasoning, then find out where the C++ code is actually spending large amounts of time, and rewrite those bits of code to something that takes less time - you can always write it like in C for these portions, avoiding all the OTHER bits that you would have to change simply because the compiler doesn't support it.

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Mats

Well realistically it appears that my only option is to go through the existing code, and find each type of item stored in a given template data structure and add each type to a union and use that as the data objects. Then I can add a bit field to the data structure to be set on initialization which would specify the type of data stored in an instance of the structure.

umm, well it's a library that needs to be integrated to work with some legacy code...

So, would it be possible to wrap the library itself in some C wrapper functions - that way, only those portions of the code that actually are exposed out of the library would need changing, rather than ALL the code.

I have done simpler porting jobs, such as translating Pascal to Modula-2 or C, and even then, it's quite hard. C++ has A LOT of things that are really difficult to replace with standard C code without some helper functionality - one option is of course to use Cfront [assuming you can find a copy that is standards compliant] to create C code - however, that would of course lead to pretty horrible and unportable code.

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Mats

Would it not be easier to convert the C code to work with the C++ library?

What you might want to do then is to create a set of wrapper functions. The template stuff might be a little difficult to do but should be possible. As long as the function signature is legal in C it really doesn't matter if the function body is C or C++.


edit: Bah I'm slow at posting today :(

Quote:

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Originally Posted by laserlight

Would it not be easier to convert the C code to work with the C++ library?

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I would imagine so, but that's not mine to tamper with

Quote:

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Originally Posted by Thantos

What you might want to do then is to create a set of wrapper functions. The template stuff might be a little difficult to do but should be possible. As long as the function signature is legal in C it really doesn't matter if the function body is C or C++.


edit: Bah I'm slow at posting today :(

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I think you are mistaken

Nope. If I have a function

Code:

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int foo(int par1, char par2, double par3);

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Then I can call it from a C program. In the body of foo I could go to town on all the C++ code I want. I simply compile the file with foo into an object file, compile the C program into object code and then link them together.

Quote:

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Originally Posted by Thantos

Nope. If I have a function

Code:

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int foo(int par1, char par2, double par3);

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Then I can call it from a C program. In the body of foo I could go to town on all the C++ code I want. I simply compile the file with foo into an object file, compile the C program into object code and then link them together.

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I see what your intention is, however linking is something that requires overhead as well. I've essentially done something similar with managed C++ and C# and I ran into some issues that were not worth while.

Quote:

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Originally Posted by gotclout

I think you are mistaken

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Well, it obviously depends on a lot of things. Maybe you can explain further why you think this is the case in your particular situation.

I can certainly say that C and C++ can be successfully mixed in various ways. You may need a bit of glue if your C-library is C only, and not C++ capable, and some restrictions may be needed to support such a thing - but it's highly likely that this work is simpler than porting existing and correct C++ code into C code. The reason being that the glue-code and restriction parts are fairly easy to identify, and a relatively small portion - whilst it's likely that almost ALL of the C++ code would have to be modified to port it to C - which leads to great risk of introducing bugs (and it's usually subtle bugs that only affect corner cases).

Where I work, we have C code that calls C++ code - the C code is a 3D graphics driver for a hardware chip (we also have a software only 3D graphics driver, also written in C), which then interfaces to the main OS which is in C++. This is an embedded OS, so it doesn't have a normal malloc/free in a C-library, and we have to actually interface the C++ low-level allocation routines (the same ones that new is based on) to make the C code have malloc and friends.

I have also worked on a Windows graphics driver that uses a C++ chip-simulator to simulate a not-yet-existing 3D graphics chip. Windows (NT and earlier at least) do not normally support C++-code in kernel drivers. So we had to work around those bits. But it's entirely possible to do, if you follow some restrictions and limitations.

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Mats

Quote:

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Originally Posted by gotclout

I see what your intention is, however linking is something that requires overhead as well. I've essentially done something similar with managed C++ and C# and I ran into some issues that were not worth while.

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Linking between two functions in different files require no more overhead then two functions in the same file. The calling method is the same.

Between C and C++ should be no problem, and as Thantos says, little performance loss (not enough to notice unless you are doing some really huge amounts of calling back and forth). C++ managed and C#, I have no idea what that does (other than being managed it has some overhead in itsel).

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Mats

To call C++ functions in C code you need to prefix your C++ functions with extern "C" so that the linkage will be compatible with C.

Code:

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extern "C" int func(int something);

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