Using/Mixing C in C++ code?

问题

Is using C in C++ bad?

Many people have told me that using C in C++ is bad because it's not as safe, and it requires more memory management. I keep telling them that as long as you know what you're doing, and you delete your "new"s and free your "malloc"s then C isn't a problem.

I'm currently on a forum where an argument over std::string vs. a char* is taking place. Some people are saying that allocating a simple char* memory block is more efficient, and as long as you deallocate it, it's fine. On the other hand we have people saying that std::string is superior because it has no memory management involved but is less efficient.

So the main question here is:

• Is mixing C/C++ bad? Should you ONLY use 100% C++ when you're coding C++?

Any answers would be appreciated!

回答1:

I keep telling them that as long as you know what your doing, and you delete your new's and free your malloc's then C isn't a problem.

This is true; if you are extraordinarily careful and ensure that you manually clean things up, then it isn't a problem. But do you really have the time to do that? Every call to new can throw std::bad_alloc. Do you always catch every exception that can be thrown and manually clean up any resources?

I'd hazard to guess the answer to that is "no," because it is very tedious to write code like that and it is difficult to be absolutely sure that code written like that is correct, even in the case of rare failures.

If the answer is "yes," then why are you wasting so much time worrying about resource management? C++ idioms like scope-bound resource management (SBRM; more commonly known as resource acquisition is initialization (RAII)) and libraries like the standard template library are there to help you write correct code more easily. Why do things the hard way when you don't have to?

Should you ONLY use 100% C++ when your coding C++?

Yes, though if there is a C library that does something you need, or if you have legacy C code that you want to use, you can certainly use that code; just be sure to be careful. Often the cleanest way to interop with C code is to write a C++ wrapper around it.

回答2:

My strong belief is that your question doesn't have to do with C or C++ at all. Your question is about trading dubious efficiency for safety. Yes, C can be more efficient. But how much more efficient? And what do you pay for that? These are the questions you should answer. In most cases string vs. const char* overhead is unnoticeable. If you are developing an efficiency-extremely-critical application, then why not code it in C in the first place?

回答3:

I'm genuinely surprised by the polarization in the answers and comments thereof.

In my eyes, the answer is pretty simple:

When writing a C++ project, use C++, avoid C ( and family) and stick to the Standard Library and STL. Ensure a homogenous C++ interface (it is a C++ project after all!) When using an external project in C, which happens to be written in C, of course you can use it. (see examples below)

Two prime examples:

1. Write a C++ program/library that does scientific calculations. I would definitely use GSL (GNU Scientific Library), and it is written in C. There are only a few caveats (like specialized initialize and free functions for specific structs and functions within GSL), that can be absorbed in a std::unique_ptr typedef. There is also the issue of error handling: checking error codes can be abstracted away in an exception mechanism if necessary/wanted, or you can keep the error codes contained within the calculation functions. GSL does have a way of setting up an error handler, I imagine some other C libraries have such a functionality.

2. Writing a C++ program using the Win32 API, which is horribly C based. I'm talking about light API usage, like reading the files in a directory, checking if a file exists, etc., not heavy GDI+ or other stuff. I like to wrap all the C functions the Win32 API exposes in nice C++ style functions, perhaps with the necessary exceptions and returning a std::string instead of having to pass a char* buffer as argument.

I understand both examples are quite... shallow... but I feel they express a general enough idea.

回答4:

Yes it's bad to mix C and C++, any the reason has nothing to do with performance or security:

It is bad cause of a maintainability:
* a C++ programmer expects all code to behave like C++.
* a C programmer expects all code to behave like C.

So when you mix C++ and C, you break both programmers expectations on how things should work.

回答5:

The better question here is, why use C? The performance? Firstly, I believe that there is no measurable performance difference for a program with the same function. Secondly, you would have to profile and prove that for your specific case, C++ is slower. Thirdly, you're giving up a huge quantity of application security by using C instead of C++.

回答6:

regrading the argument over std::string vs char *.

std::string is not going to be slower that char * (for heap char*); many implementations are much faster because they use private memory pool. And anyway the robustness of std::string far outweighs any (unlikely) perf hit

回答7:

The answer is, of course: it depends.

Generally you want to avoid mixing things that can cause confusion, that can further lead to hard-to-find bugs. Just because "you know what your doing" doesn't mean that the next person to touch your code will know what you were doing. If you're developing code that only you will ever use, then it's probably okay, but that's rarely the case.

Using C for performance is fine if you're careful. But you should only do it if you KNOW that you need the performance. Premature low-level optimization is the work of the devil.

It's a very rare case where using char* over std::string will give you any noticeable performance benefits, and it's only worth the memory management hassle in those cases where it does for sure.

回答8:

The simple answer here is, profile; determine which works best in your case and use it wisely!

回答9:

Is using C in C++ bad?

although a subjective question: in my opinion, take great measures to avoid using c in c++ programs.

Many people have told me that using C in C++ is bad because it's not as safe, and it requires more memory management. I keep telling them that as long as you know what your doing, and you delete your new's and free your malloc's then C isn't a problem.

you're reintroducing deficiencies and dangers c++ was designed to overcome, and it's not the way things are done in c++ programs.

i routinely check/reject/rewrite code that enters codebases that is "c with c++ features", or "c++ with c features". i even go as far as to change malloc, free, etc. to assert in root namespaces (among other things).

I'm currently on a forum where an argument over std::string vs. a char* is taking place. Some people are saying that allocating a simple char* memory block is more efficient, and as long as you deallocate it, it's fine. On the other hand we have people saying that std::string is superior because it has no memory management involved but is less efficient.

there are more options for representing a string in c++ than std::string.

in my opinion, it's completely valid to create a new class which represents a string and serves a particular purpose, or follows additional contracts (when necessary). part of the contracts of such string representations are (of course) that they manage their own resources using new[]/delete[] when dynamic memory is used.

if efficiency is that important and std::string is less than ideal for a specific task, then c++ is powerful enough to express your intent for these specific cases by creating a specialized interface in c++. there are plenty of cases where this is acceptable (imo), but not always worth the time investment. in any event, it's easier to manage than integrating c idioms/styles/dangers into c++ programs.

So the main question here is: Is mixing C/C++ bad? Should your ONLY use 100% C++ when your coding C++?

it's best to create reusable object based solutions for your needs. the dangers in the example provided can be completely encapsulated (if this optimization is truly worth the time investment), and be written to use c++ idioms, without performance loss and with better maintainability.

回答10:

In the specific case of string versus const char *, you should use bare const char * for all variables that hold string constants (and only string constants), converting to string only when passing to an API that requires string. Doing this consistently can eliminate enormous numbers of global constructors, does not cause memory allocation headaches (since string constants are permanent, constant data) and IMO actually makes the code clearer - you see const char *, you know that's gonna be a string constant.

回答11:

If your'e talking about techniques, I'd be careful to say that doing the above is okay.

Writing C++ programs using C-style program organization techniques will likely result in lots of maintainability issues. In general, the programmer is ignoring many of the benefits that an object-oriented language provides. Simply because much of C syntax is valid C++ and many coding techniques transfer does not mean you should do them in C++.

That said, using C functions and things isn't a problem, so long as your'e careful about their use. In some cases, you have to.

回答12:

Well, I'm in a strange situation. I am working on a system that is supposed to be in C++ (C++ compiler is used and term C++ is used), but everything is written in C. This is very frustrating, because it is getting to a point where I have to 'prove' C++ is better to use than C, even though we are coding in C++. It was all hell when I introduced std::string. My take on it is that everything now is starting to get cluttered (mixing of C and C++). There are rare instances of error handling. In fact, I think I can count 3 system wide try-catch statements. The code is messy, memory leaks are prominent and finding an error is a needle in a haystack. There are hundreds of lines of code that can be replaced by C++ functions. I'd say writing the code is more efficient, cleaner and easier to understand.

From my experience, yes of course you can mix C and C++. You can pretty much do whatever you want, but maintaining, debugging and actually figuring out what's going on becomes a problem. It's easy to say you're going to clean-up your memory allocations, but maybe someone else uses your code and doesn't. Maybe you forget to do it and you waste hours on finding silly bugs, instead of using that time to do something productive. I don't even think there should be an argument. When you do C++, do C++. When you do C, do C.

来源：https://stackoverflow.com/questions/4025869/using-mixing-c-in-c-code