2020-03-01

by Andre Leiradella

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I’m interested in hearing about your Lua/C usage! While this is a C article, I think you can implement most of the things mentioned here in your favourite language. I’ll use Lua C API and LuaBridge for examples, so I recommend to read the following articles if you’re not familiar with them: Using Lua with C (Part 1) Using Lua with C. Embedding Lua in C#.Net – Part III. This post contains a full managed lua wrapper around the lua API, a simple script engine making life easier when communicating with lua, a sample class ‘NPCPlayer’ to integrate with lua, a sample program using the script engine and the sample class and last a basic explanation of the lua stack.

Lua provides the luaL_requiref function to allow the embedding of modules, either native or written in Lua, into the executable instead of relying on the file system and search paths to locate and load modules into a Lua state.

Besides the Lua state, luaLrequiref needs the module name as a C string, and a lua_CFunction argument that is responsible for providing the module. Its signature is:

Tired: `luaLrequiref`

luaLrequiref works by checking if a module with the name passed in modname was already loaded and, if not, by calling the openf function and setting the module as the result of that call. It can also set a global variable with modname with the result of the call when glb is not zero:

To use luaL_requiref with modules written in Lua, an intermediary function to compile the Lua source code is needed. Since luaL_requiref passes modname to openf, it’s trivial to write such function:

This is all easy and works as intended, but introduces a dependency problem when many modules are embedded, as some modules may depend on other modules. The problem is that when a module is required via luaL_requiref, all the modules that it requires must be already known to the Lua state, otherwise require will fail to load the module.

The only way around this issue is calling luaL_requiref in a order that satisfies these dependencies, so that when a module is loaded, all modules that are dependencies of that module were already loaded.

Although the correct order of the luaL_requiref calls is not difficult to achieve, another problem with this approach is that all modules are forcibly loaded even though the Lua code ends up not requiring all of them.

Wired: register our own searcher

In Lua, a searcher is a function that is responsible for locating a module by its name and loading it. When created, a Lua state already comes with four searches defined by default.

It’s possible to add new searchers to the list and load modules in ways not supported by the default loaders. So instead of using luaL_requiref to pre-load all modules that we want to embed in the application, we can register a search that knows how to locate and load them.

Lua Example Code

The code below registers a searcher that can load any module from the LuaSocket library:

Now when someone requires any LuaSocket module, our searcher function will end up being called, and will return the function that defines the module. If that module in its turn requires another module, the searcher function will be called again and all the dependencies will be satisfied without requiring manual ordering of the dependencies.

Also, only the modules effectively used by the Lua code will be loaded into the Lua state.

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If the configuration file is uncertain, you have to rely on scripts. Lua is almost the first choice for C + + programs to embed embedded scripts.

Lua’s source code comes with itMakefileThe static library, interpreter and compiler can be compiled. As the host C + + program, in addition to the Lua header file, it should also link the static library.

If C + + programs are built by cmake, it is not difficult to create a static library for Lua with cmake. Cmake solves the problem of cross platform.

How To Embed Lua

In fact, there are only two problems with script extension: first, how can Lua access C + + objects? 2、 How to let C + + access Lua objects? Of course, the so-called object is a broad concept, including variables, functions, classes, and so on.

Through luabridge, these two problems can be easily solved.

Header file

I’ll give you the header file first, and I won’t mention it later.
First of all, it contains several header files of lua. Because it is C code, it is placed in theextern 'C'In order to mix with C + + program.

The second is luabridge header file. Luabridge, like STL, has only header file, which directly contains and uses.

Lua visits C++

function

C + + functionsSayHelloExport as Lua functionsayHelloAnd then through theluaL_dostringExecute Lua code and call this function.

Output:

bySayHelloAdd a parameter:

Embedding Lua In C++ Example Pdf

Output:

class

The C + + class is exported to Lua’s table, and the member function of the class corresponds to the member of the table. Suppose there is a classLine, represents a line in the text file:

Constructor

Export constructor withaddConstructor, export member functions or useaddFunction：

Constructor cannot address, calladdConstructorYou need to pass a template parameter to indicate the type.

Test:

Output:

If you have multiple constructors, such as a default constructor:

Only one can be exported. In this way, the second will cover the first:

You can’t make the same name refer to two things.

Member function

Consider a slightly more complex member function,StartWithTo determine whether a line of text begins with a string, the parameterignore_spacesDetermines whether to ignore spaces at the beginning of a line. Those that are not interested in implementation can be completely ignored.

adoptaddFunctionExport to Lua:

Test:

Output:

Output parameters

Now forStartWithAdd optional output parameters so thatignore_spacesbytrueThe offset information (subscript of the first non empty character) can be returned when the

Output parameters are very common in C / C + +, which can make a function return multiple values. But withaddFunctionExportedStartWithIt cannot be called by Lua because Lua has no output parameters. Fortunately, Lua’s functions can have multiple return values, so thatStartWithReturn multiple values, we have to do a layer of luaCFunctionPackaging.

The type isint (*) (lua_State*)The function is called LuaCFunction。 change to the use of sth.addCFunctionexportLua_StartWith：

Embedding Lua In C++

Test:

Output:

Variable parameter

Now that it’s doneCFunctionIt’s better to do it more thoroughly. In view of lua’s good support for variable parameters, we letstartWithSupport variable parameters. For example, you can judge whether the'if'Lead:

We can also judge whether the'if'or'else'Lead:

For this reason,ignore_spacesIt becomes the first parameter, followed by the variable parameter of string type. The specific implementation is as follows:

Test:

Output:

Execute Lua file

The previous example executes Lua code and uses all of themluaL_dostringIn the actual project, Lua code mainly exists in the form of file, so it is necessary toluaL_dofile。

Test:

filetest.luaThe contents are as follows:

Output:

C + + accessing Lua

adoptgetGlobalThe global Lua object of typeLuaRef。

Output:

character string

Lua doesn’t have a character type, and it doesn’tUnicodeString (specificallywchar_t*）。

Tired: luaLrequiref

Wired: register our own searcher

Lua Example Code

How To Embed Lua

Header file

Lua visits C++

function

Embedding Lua In C++ Example Pdf

class

Constructor

Member function

Output parameters

Embedding Lua In C++

Variable parameter

Execute Lua file

C + + accessing Lua

character string

Tired: `luaLrequiref`