CodingStandards

From Wesnoth

Wesnoth uses C++ that is portable to C++ compilers targeting various commonly used platforms.

C++ version

Wesnoth uses C++ conforming to C++98/03. At the moment C++11 compiler support is still not widely available, therefore C++11 is not allowed.

Formatting

When working on C++ for Wesnoth, indent your code with a tab character. After fully indenting, if you still need to line up the text with a specific character on the line above, you may further align it using space characters.

You may use long lines.

Evil things to avoid

Avoid implicit conversions

Make all constructors which only take one argument that is of a different type to the class explicit.

Do not use operator T() (where T is a type) to allow an implicit conversion to a different type. For example:

t_string(const std::string&);

This can cause many situations where a temporary t_string is implicitly created and then gets destroyed unexpectedly (reference bug #20360).

Do not declare class data members as non-private

It's okay to have a struct with only public members, if that's what you want.

However, once something is a class with private data members, do not add public (or even protected) data members to the class. Doing this breaks encapsulation and can cause all kinds of confusing and evil things to happen.

Destructors must not throw exceptions

Do not allow exceptions to propogate from a destructor. Doing that is always bad in C++. Any code which does it should be treated as a bug and fixed. Doing this is a very easy way to cause memory leaks and crashes.

It's okay to have exceptions thrown inside a destructor, just as long as you catch() them inside the destructor too and don't allow them to propagate out.

In C++11, any destructor which throws an exception causes the program to be immediately terminated (except in special cases).

You can read more about the issue here: http://c2.com/cgi/wiki?BewareOfExceptionsInTheDestructor

Naming

End non-public class data members with an underscore

All non-public data members of classes should have their names terminated with an underscore, to show that they are a class member. This makes for more readable code, once one is familiar with the convention.

Idioms

Use references when a value may not be NULL

If a value passed to a function can never be NULL, use a reference instead of a pointer. For example:

void my_function(T& obj);

rather than

void my_function(T* obj);

This more clearly shows prospective users of the function that obj may never be NULL, without them having to consult documentation or the implementation of the function.

Use the const keyword

The const keyword in C++ allows interfaces to more clearly specify how they treat objects. Always use const when you are not going to modify an object. For example:

void my_function(const T& obj);

This shows to the caller that obj will not be modified. If my_function() may modify obj, then use the following instead:

void my_function(T& obj);

Likewise, if a variable is not changed after initialization, make it const, and mark member functions as const if they do not modify their object.

Know the behavior of const references when types differ

If you assign something to a const reference of a different type, if necessary (if the type is different but there is a conversion) the compiler will create a temporary and guarantee it lasts for the lifetime of the reference. So

char c = 0;
const int& i = c;
c = 5;

will result in c == 5 and i == 0, which may not be what you expect.

Write exception-safe code

Wesnoth code should be exception-safe, even if you do not use exceptions directly. That is, you should be able to assume that an exception is thrown almost anywhere from within the code, with well-defined results (i.e. no resource leaks).

Code that uses a pattern like the following is bad:

{
    SDL_Surface* image = IMG_Load("image.bmp");
    ...some code, which uses 'image'...
    SDL_FreeSurface(image);
}

The code may throw an exception, and image will never be freed. Instead, use wrapper objects which free the object in their destructor.

For SDL_Surface objects, the surface type is used throughout the Wesnoth source code to achieve this purpose. So you could rewrite the above code as follows:

{
    surface image(IMG_Load("image.bmp"));
    ...some code, which uses 'image'...
} the image is automatically freed here when 'image' is destroyed

Instead of allocating memory directly using new[] or malloc(), use language-provided containers, such as vector.

Similarly, avoid writing delete explicitly in your code. Oftentimes using a smart pointer or similar instead will improve the readability of your code, by making it obvious that you are managing memory correctly even if an exception is thrown. If you were deleting a pointer which is a member variable of an object, using a smart pointer instead may simplify your code by eliminating the need to write an explicit destructor in some cases.

For more information, you can read about the (important) RAII idiom: http://c2.com/cgi/wiki?ResourceAcquisitionIsInitialization

Do not use sprintf

The sprintf() function does not check whether or not it is writing past the end of the space allocated. This is a security problem if someone other than the person running the game can cause sprintf() to write very long strings. In Wesnoth, this untrusted data could come potentially from other players in a multiplayer game, or from downloaded add-ons. Instead you should use snprintf() with the second argument being the sizeof of the buffer that will hold the result.

Standard C++ to avoid

Do not use wstring

The standard C++ std::wstring class (defined as a std::basic_string< wchar_t >) does not exist in some platforms supported by Wesnoth. Use wide_string instead (defined in language.hpp). The wide_string type is actually defined as std::vector< wchar_t >.

You may use std::wstring in an #if that guarantees that wstring is supported on all supported plattforms that don't get filtered out by the #if.

Do not use 0 when you mean NULL

Several Wesnoth developers, including Dave, find the number 0 to be very ambiguous when used in a non-numeric context. In keeping with the precedent that has already been established in the Wesnoth source code, you should avoid using literal zero for initializing and/or comparing null pointers.

C legacy to be avoided

Use util::array instead of C-style Arrays

C-style arrays are very efficient, but their interface is ugly. Use util::array defined in array.hpp instead. It is a wrapper for an array which has a C++ container-style interface. If you need to, extend it to make it fit your needs.

Do not use C-style casts

The following code,

if(i->second.side() == (size_t)player_number_) {

is considered bad practice in C++ since a C-style cast is overpowered -- if types change around it could end up casting away constness, or performing an implementation-defined data reinterpretation (basically a C-style cast is a compiler-generated combination of static_cast, reinterpret_cast, and const_cast).

Good programming style is to use the least powerful tool available that does what you want. For example:

if(i->second.side() == static_cast<size_t>(player_number_)) {

Alternatively, a constructor call may be used for non-built-in types.

Note: there may be some obscure cases where a C-style cast is desirable, such as converting a pointer to an integer type of unspecified size.

Do not use #define for constants

#define foo X is not a typesafe approach to define constants. Instead, you can something like the following (in an anonymous namespace) to achieve the same goal in a typesafe fashion.

namespace {
    const T foo = X;
}

Documentation

Document config preconditions and postconditions

In the Wesnoth code you will commonly encounter a data container type known as config, which contains hierarchical string data (such as WML contents or game settings). The tagged children of the config object and their string attributes are arranged in an ordered and mapped format, internally implemented using the C++ STL.

Because config data is utilized in so many ways and places, it can be difficult to track across the scope of the entire program. Thus, you should document all public functions that take/return config objects, specifying content expectations and updating any related entries in the ReferenceWML wiki pages. In particular, if your function requires a config parameter, specify where/how the config object should be created. This will be a great help to any future coders who need to call or modify your function.

Doxygen

See Doxygen for tips on how to comment the code, so that Doxygen can nicely document it.

See also

This page was last modified on 17 December 2014, at 03:48.