I am very new in visual c++ and I meet a problem in my development with some provided API(.lib).
Here's my code:
in header file
ref class RFID{
public:
int connect(char* p);
private:
HANDLE port;
}
in cpp file
#include "stdafx.h"
#include "RFID.h"
int RFID::connect(char* p){
return RmuOpenAndConnect(port,p,0);
}
The error line is:
error C2664: 'RmuOpenAndConnect' : cannot convert parameter 1 from 'HANDLE' to 'HANDLE &'
Since I am very new in visual c++ and I don't know how to solve with this error, it seems that the parameter is not just an address of "HANDLE" so that I don't know how to soft it.
Thanks for any help.
Edit:
Sorry that I add the details in comment.
In header file, RmuOpenAndConnect is defined as following:
int WINAPI RmuOpenAndConnect (HANDLE &hCom, char* cPort, UCHAR flagCrc);
Because I want to use Thread in my program(http://msdn.microsoft.com/en-us/library/system.threading.parameterizedthreadstart.aspx), and it seems the class should be a "ref class" so I did.
This is because HANDLE port is member of managed class, which is subject of garbage collection. Reference to managed class member cannot be used, because class instance can change its address. You can use local variable to fix this:
int RFID::connect(char* p){
HANGLE h = port;
int n = RmuOpenAndConnect(h,p,0);
port = h; // in the case RmuOpenAndConnect changed the handle
return n;
}
Related
I'm trying to do something very simple... declare a string in a class, and then assign it to the value of another string defined in a class constructor.
I am using a managed wrapper for an unmanaged class "Unmanaged" (using a managed wrapper because I want to use it in a C# program, and something I am using is unmanaged and its .sln file is not under my control)
As you can see, I tried including as many string headers as possible.
#pragma once
#include <string>
#include <string.h>
#include <cstring>
#include <iostream>
using namespace std;
using namespace System;
using std::string;
namespace UnmanagedWrap {
public ref class Class1
{
// TODO: Add your methods for this class here.
public:
Unmanaged *pu; //pointer to the Unmanaged class
//the constructor will allocate the pointer pu
int a;
int b;
std::string filePath; //try CString() when get back
Class1(int a_In, int b_In, std::string filePath_In) : pu(new Unmanaged()) { //constructor
a = a_In;
b = b_In;
filePath = filePath_In; //trying to assign filePath to the inputted filePath_In.......
}; //end of constructor
This is giving me 2 errors:
The first relates to the line std::string filePath;
1>c:\users\ngrace\documents\visual studio 2010\projects\unmanagedwrap\unmanagedwrap\UnmanagedWrap.h(21): error C4368: cannot define 'filePath' as a member of managed 'UnmanagedWrap::Class1': mixed types are not supported
The second relates to the line filePath = filePath_In;
1>c:\users\ngrace\documents\visual studio 2010\projects\unmanagedwrap\unmanagedwrap\UnmanagedWrap.h(25): error C2678: binary '=' : no operator found which takes a left-hand operand of type 'std::string' (or there is no acceptable conversion)
I am very lost as I have spent hours searching for an answer...
Some pages I have gone to for help:
Including headers from an unmanaged C++ code inside C++/CLI code
Mixed types are not supported
(I would post more but I need reputation of at least 10 to do so....)
Any ideas on why I'm getting these errors?
Using information from Bo Persson and user2666293, I was led to try something, which ended up being an answer to this.
You must use System::String^ type for managed strings. If using a managed string and passing it to a method in an unmanaged class, it must be converted to an unmanaged string type!
Let's say we're using the unmanaged string type std::string in the unmanaged class.
The conversion from System::String^ to std:string must be done using:
auto unmannedString = msclr::interop::marshal_as<std::string>(managedString);
and with a header file reference at the top of:
#include <msclr/marshal_cppstd.h>
where managedString is of type System::String^
:)
https://msdn.microsoft.com/en-us/library/hh699870.aspx says under bullet 2 that ref classes can only have standard c++ types if they are not public. Try using a regular class or making filePath private.
edit:
I'm not familiar with managed classes but you might try Platform::string instead of std::string : https://msdn.microsoft.com/en-us/library/hh755812.aspx
https://msdn.microsoft.com/en-us/library/hh699879.aspx
says to use platform strings when you pass strings back and forth to methods in Windows Runtime classes, or when you are interacting with other Windows Runtime components
I am trying to link to an external library in my QT application. The external library has a header file with the following relevant code I'm trying to call:
extern VGRABDEVICE_API bool V_AssignFrameSizeCallback(IGrabChannel* pChannel, void* pFunc);
In the demo C++ program provided, which has no problems compiling, the following relevant code is:
// in main.cpp
void _stdcall MyFrameSizeCallback(T x) {
do_stuff;
}
int main(int argc, char* argv[]) {
IGrabChannel* pChannel0 = something;
V_AssignFrameSizeCallback(pChannel0, MyFrameSizeCallback);
}
I am trying to incorporate this code into my QT application, but getting problems. In my mainwindow.cpp file:
void _stdcall MainWindow::MyFrameSizeCallback(T x) {
do_stuff;
}
void MainWindow::someFunction() {
IGrabChannel* pChannel0 = something;
V_AssignFrameSizeCallback(pChannel0, &MainWindow::MyFrameSizeCallback);
}
The error I'm getting is:
error: C2664: 'bool V_AssignFrameSizeCallback(IGrabChannel *,void *)' :
cannot convert argument 2 from 'void (__cdecl MainWindow::* )(T)' to 'void *'
There is no context in which this conversion is possible
What do I need to do? Thanks.
You have two problems. First, void* is a data pointer, not a function pointer. According to the C++ standard, casting between the two is not expected to work. Some platforms provide a stronger guarantee... for example Windows GetProcAddress and *nix dlsym mix the two.
Next, your &MainWindow::MyFrameSizeCallback is not a function pointer, it is a pointer-to-member-function. Calling it requires a MainWindow object, which the external library doesn't know anything about.
You need to provide an ordinary function, not a member function, to the library. If you have some way to get ahold of the MainWindow* object pointer, you can then call its member function to do the real work. Sometimes the library provides a "context" parameter which is passed to your callback; that's a great place to put the object pointer. Otherwise, you'll need to store your MainWindow* in a global variable. Easy if you have just one, while if you have more than one you might go with std::map<IGrabChannel*, MainWindow*>.
Code:
MainWindow* MainWindow::the_window;
void MainWindow::MyFrameSizeCallback(T x)
{
do_stuff;
}
void _stdcall MyFrameSizeCallbackShim(T x)
{
MainWindow::the_window->MyFrameSizeCallback(x);
}
void MainWindow::someFunction()
{
IGrabChannel* pChannel0 = something;
the_window = this;
V_AssignFrameSizeCallback(pChannel0, &MyFrameSizeCallbackShim);
}
If the parameter x isn't an IGrabChannel, change the map datatype and insertion logic accordingly. If the parameter x isn't some sort of unique predictable identifier, you may be limited to only doing callbacks to one MainWindow instance.
I'm using Visual Studio 2012 with SDL and I'm doing a simple threading task but I always get these errors:
argument of type "int (mainGame::*)(void *ptr)" is incompatible with parameter of type "int (__cdecl *)(void *)"
The other error I'm getting:
error C3867: 'mainGame::gameEvents': function call missing argument list; use '&mainGame::gameEvents' to create a pointer to member
This is how the function is written:
int gameEvents(void *ptr){
//do things here.
return 0;
}
This is the code I'm using to call the function:
SDL_Thread* gh;
gh = SDL_CreateThread(gameEvents,NULL);
Since you're using C++, you need to declare your function with C linkage, since that's what SDL expects, being a C library itself:
extern "C" int gameEvents(void *ptr);
Change gameEvents() to static. Otherwise your member function will have a "hidden" first argument for the this pointer that C APIs like SDL don't know about.
If you need access to instance data do something like this:
static int mainGame::gameEvents(void *ptr)
{
mainGame* game = (mainGame*)ptr;
//do things here.
return 0;
}
...
mainGame game;
SDL_Thread* gh;
gh = SDL_CreateThread( mainGame::gameEvents, &game );
If I instantiate a mapped_file_source (boost 1.46.1 ) with a narrow character string as in the following I don't have a problem:
boost::iostreams::mapped_file_source m_file_( "testfile.txt" );
However if I try to use a wide string:
boost::iostreams::mapped_file_source m_file_( L"testfile.txt" );
I get the following compiler error in VC2010 SP1:
P:\libs\boost_1_46_1\boost/iostreams/device/mapped_file.hpp(128): error C2248: 'boost::iostreams::detail::path::path' : cannot access private member declared in class 'boost::iostreams::detail::path'
P:\libs\boost_1_46_1\boost/iostreams/detail/path.hpp(111) : see declaration of 'boost::iostreams::detail::path::path'>
P:\libs\boost_1_46_1\boost/iostreams/detail/path.hpp(37) : see declaration of 'boost::iostreams::detail::path'
If I instead try to pass the constructor a boost::filesystem::path I get the following error:
P:\libs\boost_1_46_1\boost/iostreams/device/mapped_file.hpp(128): error C2664: 'boost::iostreams::detail::path::path(const std::string &)' : cannot convert parameter 1 from 'const boost::filesystem3::path' to 'const std::string &'
Reason: cannot convert from 'const boost::filesystem3::path' to 'const std::string'
I feel like I'm missing something obvious, but I'm just running around in circles trying to figure out what the compiler is trying to tell me, but I'm just getting lost. That palm to forehead moment is just not happening.. What is it that I am doing incorrectly?
The constructor defined in mapped_file.hpp looks like the following:
// Constructor taking a parameters object
template<typename Path>
explicit mapped_file_source(const basic_mapped_file_params<Path>& p);
The basic_mapped_file_params class constructors look like this:
// Construction from a Path
explicit basic_mapped_file_params(const Path& p) : path(p) { }
// Construction from a path of a different type
template<typename PathT>
explicit basic_mapped_file_params(const PathT& p) : path(p) { }
Where the template class is defined as:
// This template allows Boost.Filesystem paths to be specified when creating or
// reopening a memory mapped file, without creating a dependence on
// Boost.Filesystem. Possible values of Path include std::string,
// boost::filesystem::path, boost::filesystem::wpath,
// and boost::iostreams::detail::path (used to store either a std::string or a
// std::wstring).
template<typename Path>
struct basic_mapped_file_params
: detail::mapped_file_params_base
{
There is some additional help in the header that says:
// For wide paths, instantiate basic_mapped_file_params
// with boost::filesystem::wpath
If I take this approach with:
boost::iostreams::basic_mapped_file_params<boost::filesystem::wpath> _tmp(L"test.txt");
boost::iostreams::mapped_file_source m_file_( _tmp );
I get the same C2664 error mentioned above..
I know the compiler is telling me what the problem is, but looking at the header source and the comments leads me to believe that what I'm trying to accomplish is supported, it's just my approach that is incorrect. Am I misinterpreting what the header file is telling me? I know there is probably a good lesson about template instantiation and explicit/implicit conversion in here somewhere.
Interestingly enough, upgrading my boost install to 1.47.0 seems to cleared up C2664 error but I'm still getting the C2248 error about access to the private member.
With boost 1.48 I can do something like this.
#include <boost/filesystem.hpp>
#include <boost/iostreams/device/mapped_file.hpp>
#include <iostream>
int main()
{
boost::filesystem::path p(L"b.cpp");
boost::iostreams::mapped_file file(p); // or mapped_file_source
std::cout << file.data() << std::endl;
}
or you can do this with mapped_file_params(used create new file)
boost::filesystem::path p(L"aa");
basic_mapped_file_params<boost::filesystem::path> param; // template param
param.path = p;
param.new_file_size = 1024;
It's telling you that boost::iostreams::mapped_file_source's constructor does not take a wchar_t*, nor does it take a boost::filesystem::path. It only takes std::string, or types convertible to std::string. Or, to put it another way, you can't use UTF-16 paths with this object.
It looks like the documentation for mapped_file is pretty old and does not reflect what is in the header or in the header comments. In order to instantiate a boost::iostreams:mapped_file_source object with a wide character string you need to explicity pass in the boost::iostreams::detail::path like this:
boost::iostreams::mapped_file_source m_file_( boost::iostreams::detail::path(boost::filesystem::path(L"testfile.txt")) );
I was able to get this to compile by stepping thought the error messages and determining how the template classes were being instantiated and finally saw that boost::iostreams::detail::path had a private constructor that took a &std::wstring as a parameter which is where the code was failing to compile.
The following for creating a Global Object is resulting in compilation errors.
#include "stdafx.h"
#include <iostream>
using namespace System;
using namespace std;
#pragma hdrstop
class Tester;
void input();
class Tester
{
static int number = 5;
public:
Tester(){};
~Tester(){};
void setNumber(int newNumber)
{
number = newNumber;
}
int getNumber()
{
return number;
}
}
Tester testerObject;
void main(void)
{
cout << "Welcome!" << endl;
while(1)
{
input();
}
}
void input()
{
int newNumber = 0;
cout << "The current number is " << testerObject.getNumber();
cout << "Change number to: ";
cin >> newNumber;
cout << endl;
testerObject.setNumber(newNumber);
cout << "The number has been changed to " << testerObject.getNumber() << endl;
}
Here are the compile errors:
1>------ Build started: Project: test, Configuration: Debug Win32 ------
1>Compiling...
1>test.cpp
1>.\test.cpp(15) : error C2864: 'Tester::number' : only static const integral data members can be initialized within a class
1>.\test.cpp(33) : error C2146: syntax error : missing ';' before identifier 'testerObject'
1>.\test.cpp(33) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\test.cpp(49) : error C2039: 'getNumber' : is not a member of 'System::Int32'
1> c:\windows\microsoft.net\framework\v2.0.50727\mscorlib.dll : see declaration of 'System::Int32'
1>.\test.cpp(55) : error C2039: 'setNumber' : is not a member of 'System::Int32'
1> c:\windows\microsoft.net\framework\v2.0.50727\mscorlib.dll : see declaration of 'System::Int32'
1>.\test.cpp(57) : error C2039: 'getNumber' : is not a member of 'System::Int32'
1> c:\windows\microsoft.net\framework\v2.0.50727\mscorlib.dll : see declaration of 'System::Int32'
1>Build log was saved at "file://c:\Users\Owner\Documents\Visual Studio 2008\Projects\test\test\Debug\BuildLog.htm"
1>test - 6 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
How do I create a Global Class
Object correctly like I've attempted
here.
And how do I fix that "only static
const integral data members can be
initialized within a class"
And basically how do I fix the rest
of the errors so I can get this to
compile?
I like declaring Global Class Objects at file scope (I like declaring all globals at file scope) because when I have to create separate source files and do "extern" and everything it becomes extremely complicated and never works for me. Although, I do want to figure out how to do that eventually... it seems every tutorial I look at won't compile though and unless it compiles I have no idea how to recreate it!
If I can just get this to compile...then I can successfully learn how to do this. So if someone could rewrite the above to where it literally copies & pastes into Visual C++ Express 2008 and works I will finally be able to figure out how to recreate it. I'm extremely excited on seeing the fix for this! It is just I can't get Global Objects to work right! Any other information on declaring Global Class Objects...or anything for that matter is welcome!
Just start addressing the errors one by one. A lot of the errors are just cascaded from the initial errors, so it looks like there are a lot of problems when there's only a couple. Just start from the top:
1>.\test.cpp(15) : error C2864: 'Tester::number' : only static const integral data members can be initialized within a class
You can't initialize a member in the class definition unless it's static, const, and one of the integral types. Leave the "= 5" off of the declaration of number. Then you'll need to have a definition of Tester::number outside of the class definition, like so:
int Tester::number = 5;
Problem #2:
1>.\test.cpp(33) : error C2146: syntax error : missing ';' before identifier 'testerObject'
Almost exactly what it says (missing semi-colon errors can be a bit inexact in saying where the semicolon should be) - you need a semi-colon after the definition of the Tester class.
Fix those and your compilation problems go away.
The key thing is to try and take compiler errors one at a time from the top. If you get more than about 3 of them, you can probably just ignore everything after the 3rd or so because the initial error just cause the compile to into the weeds (and if they are real errors, they'll show up again in the next compile anyway).
Error C2864: either add a const modifier to your integer, or move the initialization outside the class (as in class Tester { static int number; }; int Tester::number = 5;). The latter seems more appropriate to your case.
Error C2146: you're missing a semicolon after the declaration of class Tester { ... }. It should be class Tester { ... };
The other errors are probably caused by the previous error. They should fix themselves automatically when it is fixed.
As a side note, I don't think you really want the static modifier on your member. It seems more appropriate for an instance field. You still can't initialize it in-place though (this isn't C#), you have to move the initialization to the constructor. For example:
class Tester {
int number;
static int staticNumber; // just to show you how to use a static field
public:
Tester() : number(5) {}
~Tester() {} // I suggest you remove the destructor unless you need it
int getNumber() { return number; }
void setNumber(int value) { number = value; }
static int getStaticNumber() { return staticNumber; }
static void setStaticNumber(int value) { staticNumber = value; }
};
// initialize static members *outside* the class
int Tester::staticNumber = 5;
According to this: http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/index.jsp?topic=/com.ibm.xlcpp8l.doc/language/ref/cplr038.htm
Tester testerObject;
int Tester::number = 5;
I'm not positive, but I think the rest of the errors come from that one problem.
Fix that, and see how far it gets you.
the answers already here deal with why your code doesn't compile and how to correct that. however i am intrigued by your comments about "extern". it is very easy to use when you know how. you declare in one header the extern'ed variable. and then you initialise it in one file. any other file can refer to the variable by including the header. e.g.
header.h:
// ensure the file is only included once
#ifndef _HEADER_H
#define _HEADER_H
extern int foo;
#endif
// end file header.h
header.cpp
#include "header.h"
int foo = 1;
// end file header.cpp
main.cpp
#include "header.h"
#include <stdio.h>
int main(int argc, char** argv)
{
printf("%d", foo);
return 0;
}
// end file main.cpp
Whilst using static class members for global variables helps fit the oo design scheme, its more elaborate than necessary. if you don't have to follow oo strictly, just use extern, its easier and its less code.