Nim language has a great FFI with C/C++ which makes it easy to use C/C++ libraries. This article introduces
how to wrapconst char* in C/C++ backend.

In the Nim world, we often play withstring which handles the dirty works for us such as resizing the buffer, creating/deleting the memory which can satisfy the most of our needs. However, if we want to interact with other languages, we need the power ofcstring. Note thatcstring doesn't mean "string in C", it means compatible string(char * in C/C++ backend, JS string in JavaScript backend and so on).

Nim doesn't provideconst char * for us. But it is easy to make our own one.

You need to importconst char* types from C/C++ backend usingimportc pragmas. Then make a distinct type for it.

typecstringConstImpl{.importc:"const char*".}=cstringconstChar*=distinctcstringConstImpl

Now, let's look at a simple example using it.

Firstemit the c function, then write the function declaration(nodecl pragmas means that we
do not generate the function declaration in the output codes). Finally we construct aconstChar object and
pass it to thefn proc.

{.emit:"""int fn(const char* a, char b) {  return 1;}""".}procfn(a:constChar,b:char):int{.importc:"fn",nodecl.}varx=constChar("abc")doAssertfn(x,'c')==1doAssertnotcompiles(fn("abc",'b'))doAssertnotcompiles(fn("abc".cstring,'b'))

We could see that thefn proc doesn't acceptstring andcstring types and only theconstChar type is allowed. After looking at the generated C codes usingnim c -r --nimcache:nimcache app.nim ornim cpp -r --nimcache:nimcache app.nim, it does generateconst char* x = "abc";.

Lastly let's look at an example with C++ backend, it has the same story as above.

Now we need to create a new.h file and overload thefn functions.

fun.h

intfn(constchar*a,charb){return1;}intfn(char*a,charb){return2;}

fun.nim

procfn(a:constChar,b:char):int{.importcpp:"$1(@)",header:"fun.h".}=discardprocfn(a:cstring,b:char):int{.importcpp:"$1(@)",header:"fun.h".}=discardvara=constChar("abs")doAssertfn(a,'b')==1varb=cstring("abc")doAssertfn(b,'b')==2

Run the codes above usingnim cpp -r fun.nim and it works!

Reference:
https://github.com/nim-lang/Nim/issues/3720

Nim is a great language. Let's port and wrap more libraries for it.

https://github.com/nim-lang/needed-libraries/issues

https://github.com/nothings/single_file_libs