c++ - How Does WinAPI ensure compilation fail when WinMain is not implemented -

-

when create win32 project in visual studio , dont define winmain function compilation error:

error 1 error lnk2019: unresolved external symbol _winmain@16 referenced in function ___tmaincrtstartup

how api setup designed/implemented ensure user can never create win32 application without implementing winmain? creating api , want put in compilation errors when user hasn't used api or hasn't implemented essential features.

an example of want use similar is: user must implement app class (base or inherited). if can somehow detect @ compilation time app class has not been implemented can throw compilation error.

is winapi's implementation simple forward declaration function hasn't been implemented (which ensures compilation failure)? ie,

// forward declaration with, intentionally, no function implementation int apientry _twinmain(_in_ hinstance  hinstance, _in_opt_ hinstance  hprevinstance, _in_ lptstr  lpcmdline, _in_ int  ncmdshow);

when starting program, os loads module , calls initialization code. symbol carries initialization code (entrypoint) can changed, typically winmaincrtstartup or maincrtstartup (if remember correctly). now, function supplied compiler vendor , linked program. first calls initialization code, e.g. constructors of globals , delegates various forms of main() or winmain(). here lies reason why compilation fails: since initialization code included (via externally visible entrypoint) , calls according main function, missing main function cause linker error.

note simple declaration not enough (you have tried that), must ensure function declares used.

concerning approach generate errors, don't think it's feasible, @ least not @ compile-time. if possible, try avoid such rigid constraints using library. why shouldn't there multiple such classes or multiple uses library inside application? if constraint makes sense, consider using technique many windowing libraries provide global function registers user-written instance. can add according checks code assert() instance registered.


Comments

Post a Comment

Popular posts from this blog

c# - How Configure Devart dotConnect for SQLite Code First? -

-
can configure following sample code ? when run following code, error : no entity framework provider found ado.net provider invariant name 'devart.data.sqlite' setting in machine.config <system.data> <dbproviderfactories> <add name="dotconnect sqlite" invariant="devart.data.sqlite" description="devart dotconnect sqlite" type="devart.data.sqlite.sqliteproviderfactory, devart.data.sqlite, version=4.6.287.0, culture=neutral, publickeytoken=09af7300eec23701" /> </dbproviderfactories> </system.data> after add below block code in app.config <?xml version="1.0"?> <configuration>   <configsections> <!-- more information on entity framework configuration, visit http://go.microsoft.com/fwlink/?linkid=237468 --> <section name="entityframework" type="system.data.entity.internal.configfile.entityframeworksection, entit...
Read more

java - Copying object fields -

-
what best way of copying classes when need have 2 independent variables? have simple class: public class myclass { boolean = false; string b= "not empty"; } do need make method : assign(myclass data ) { a= data.a; b= data.b; } is there automatic method copy (duplicate) objects in java? do need make method : pretty close. instead of making method, should make constructor. such constructors called copy constructor , , create them this: myclass(myclass data) { = data.a; b = data.b; } and create copy of instance, use constructor this: myclass obj1 = new myclass(); myclass obj2 = new myclass(obj1); copy constructor can tedious : using copy constructor create deep-copy can tedious, when class has mutable fields. in case, assignment create copy of reference, , not object itself. have create copy of fields (if want deep copy). can go recursive. a better way create deep copy serialize , deserialize object. why not use clone()? ...
Read more

c++ - Clear the memory after returning a vector in a function -

-
i have function processes , stores lot of data in , returns results vector of class. amount of data stored in function tremendous , want clear storage memory of function after finished job. necessary (does function automatically clear memory) or should clear memory function? update: vector<customers> process(char* const *filename, vector<int> id) { vector<customers> list_of_customers; (perform actions) return list_of_customers; } variables defined locally within function automatically released @ end of function scope. destructors objects called, should free memory allocated objects (if objects coded correctly). example of such object std::vector . anything you've allocated new must have corresponding delete release storage. try avoid doing own allocation , use raii instead, i.e. containers or smart pointers.
Read more