Walkthrough: Creating and Using a Dynamic Link Library (C++)

Artículo
12/18/2017

The latest version of this topic can be found at Walkthrough: Creating and Using a Dynamic Link Library (C++).

This step-by-step walkthrough shows how to create a dynamic link library (DLL) for use with a C++ app. Using a library is a great way to reuse code. Rather than re-implementing the same routines in every program that you create, you write them one time and then reference them from apps that require the functionality. By putting code in the DLL, you save space in every app that references it, and you can update the DLL without recompiling all of the apps that use it. For more information about DLLs, see DLLs in Visual C++.

This walkthrough covers these tasks:

Create a DLL project.
Add a class to the DLL that exports static functions.
Create a console app project.
Use the functions exported by the DLL in the console app.
Run the completed app.

This walkthrough creates a DLL that can be called from apps that use C++ calling conventions. This requires both the DLL and the client app to be built by using the same compiler toolset, so that the internal naming conventions match. It's also possible to create DLLs that can be called from apps written in other languages and built using other compilers by using the C calling convention. For more information about specifying C linkage, see Exporting C++ Functions for Use in C-Language Executables. For information about how to create DLLs for use with other languages, see Calling DLL Functions from Visual Basic Applications.

This simple walkthrough uses a combined solution that contains both the DLL and the client app, and uses implicit linking at load-time to import the DLLs functions. A more common situation involves third-party DLLs that are not built as part of your solution, or that use explicit linkage to load the DLLs at run-time rather than at load-time. For more information about implicit linking and explicit linking, see Determining Which Linking Method to Use.

Prerequisites

This topic assumes that you understand the fundamentals of the C++ language and the basics of using the Visual Studio IDE. The Visual C++ components must be installed in Visual Studio to use this walkthrough.

To create a dynamic link library (DLL) project

On the menu bar, choose File, New, Project.
In the left pane of the New Project dialog box, expand Installed, Templates, Visual C++, and then select Win32.
In the center pane, select Win32 Console Application.
Specify a name for the project—for example, MathLibrary—in the Name box. Specify a name for the solution—for example, MathLibraryAndClient—in the Solution name box. Choose the OK button.
On the Overview page of the Win32 Application Wizard dialog box, choose the Next button.
On the Application Settings page, under Application type, select DLL.
Choose the Finish button to create the project.

To add a class to the dynamic link library

To create a header file for a new class, on the menu bar, choose Project, Add New Item. In the Add New Item dialog box, in the left pane, select Visual C++. In the center pane, select Header File (.h). Specify a name for the header file—for example, MathLibrary.h—and then choose the Add button. A blank header file is displayed.
Replace the contents of the header file with this code:
```
// MathLibrary.h - Contains declaration of Function class  
#pragma once  

#ifdef MATHLIBRARY_EXPORTS  
#define MATHLIBRARY_API __declspec(dllexport)   
#else  
#define MATHLIBRARY_API __declspec(dllimport)   
#endif  

namespace MathLibrary  
{  
    // This class is exported from the MathLibrary.dll  
    class Functions  
    {  
    public:  
        // Returns a + b  
        static MATHLIBRARY_API double Add(double a, double b);  

        // Returns a * b  
        static MATHLIBRARY_API double Multiply(double a, double b);  

        // Returns a + (a * b)  
        static MATHLIBRARY_API double AddMultiply(double a, double b);  
    };  
}  
```
This code declares a namespace, MathLibrary, that contains a class named Functions that contains member functions to perform some mathematical operations.

Notice the preprocessor statements at the top of the file. By default, the New Project template for a DLL adds PROJECTNAME_EXPORTS to the defined preprocessor symbols for the DLL project. In this example, MATHLIBRARY_EXPORTS is defined when your MathLibrary DLL project is built. When the MATHLIBRARY_EXPORTS symbol is defined, the MATHLIBRARY_API symbol sets the __declspec(dllexport) modifier on the member function declarations in this code. This modifier tells the compiler and linker to export the function or variable from the DLL so that it can be used by other applications. When MATHLIBRARY_EXPORTS is undefined—for example, when the header file is included by a client application—MATHLIBRARY_API defines the __declspec(dllimport) modifier on the member function declarations. This modifier optimizes the import of the function in an application. For more information, see dllexport, dllimport.

Note

If you are building the DLL project on the command line, use the /D compiler option to define the MATHLIBRARY_EXPORTS symbol.
In the MathLibrary project in Solution Explorer, in the Source Files folder, open MathLibrary.cpp.

Implement the members of the Functions class in the source file. Replace the contents of the MathLibrary.cpp file with the following code:

// MathLibrary.cpp : Defines the exported functions for the DLL application.  
// Compile by using: cl /EHsc /DMATHLIBRARY_EXPORTS /LD MathLibrary.cpp  

#include "stdafx.h"  
#include "MathLibrary.h"  

namespace MathLibrary  
{  
    double Functions::Add(double a, double b)  
    {  
        return a + b;  
    }  

    double Functions::Multiply(double a, double b)  
    {  
        return a * b;  
    }  

    double Functions::AddMultiply(double a, double b)  
    {  
        return a + (a * b);  
    }  
}

To verify that everything is working so far, compile the dynamic link library by choosing Build, Build Solution on the menu bar. The output should look something like this:
```
1>------ Build started: Project: MathLibrary, Configuration: Debug Win32 ------  
1>  stdafx.cpp  
1>  dllmain.cpp  
1>  MathLibrary.cpp  
1>     Creating library c:\users\username\documents\visual studio 2015\Projects\MathLibraryAndClient\Debug\MathLibrary.lib and object c:\users\username\documents\visual studio 2015\Projects\MathLibraryAndClient\Debug\MathLibrary.exp  
1>  MathLibrary.vcxproj -> c:\users\username\documents\visual studio 2015\Projects\MathLibraryAndClient\Debug\MathLibrary.dll  
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========  
```
Note

If you are using an Express edition that does not display a Build menu, on the menu bar, choose Tools, Settings, Expert Settings to enable it, and then choose Build, Build Solution.

Note

If you are building a project on the command line, use the /D compiler option to define your project's PROJECTNAME_EXPORTS preprocessor symbol. Use the /LD compiler option to specify that the output file is to be a DLL. For more information, see /MD, /MT, /LD (Use Run-Time Library). Use the /EHsc compiler option to enable C++ exception handling. For more information, see /EH (Exception Handling Model).

Congratulations, you've created a DLL using Visual C++! Next, you'll create a client app that uses the functions exported by the DLL.

To create an app that references the DLL

To create a C++ app that uses the DLL that you just created, on the menu bar, choose File, New, Project.
In the left pane of the New Project dialog, expand Installed, Templates, Visual C++, and then select Win32.
In the center pane, select Win32 Console Application.
Specify a name for the project—for example, MathClient—in the Name box.
Choose the drop-down button at the end of the Solution control, and then select Add to Solution from the drop-down list. This adds the new project to the same solution that contains the DLL. Choose the OK button.
On the Overview page of the Win32 Application Wizard dialog box, choose the Next button.
On the Application Settings page, under Application type, select Console application.
Choose the Finish button to create the project.

To use the functionality from the class library in the app

When the Win32 Application Wizard finishes, a minimal console application project is created for you. The name for the main source file is the same as the project name that you chose earlier. In this example, it is named MathClient.cpp.
To use the math routines that you created in the DLL, you must reference the DLL in your app. To do this, under the MathClient project in Solution Explorer, select the References item. On the menu bar, choose Project, Add Reference.

Note

In older versions of Visual Studio, references are added to your project in a different way. Select the MathClient project in Solution Explorer, and then on the menu bar, choose Project, References. In the Property Pages dialog box, expand the Common Properties node, select Framework and References, and then choose the Add New Reference button. For more information about the References dialog box, see Adding references.
The Add Reference dialog box lists the libraries that you can reference. The Projects tab lists the projects in the current solution and any libraries that they contain. On the Projects tab, select the check box next to MathLibrary, and then choose the OK button.
You need the definitions in the MathLibrary.h file to call the DLLs functions from your app. You could copy the header file into your client app project, but that might lead to changes in one copy that are not reflected in the other. To avoid this issue when you reference the header files of the DLL, you can change the included directories path in your project to include the original header. To do this, open the Property Pages dialog box for the MathClient project. In the left pane, expand Configuration Properties, C/C++ node, and then select General. In the center pane, select the drop-down control next to the Additional Include Directories edit box, and then choose <Edit...>. Select the top pane of the Additional Include Directories dialog box to enable an edit control. In the edit control, specify the path to the location of the MathLibrary.h header file. Because typing the complete path may be difficult, you can use the browse control (...) at the end of the edit box to bring up a Select Directory dialog box. In the dialog, navigate up one folder level to the MathLibraryAndClient folder, then select the MathLibrary folder, and then choose the Select Folder button. Once you've entered the path to the header file in the Additional Include Directories dialog box, choose the OK button to go back to the Property Pages dialog box, and then choose the OK button to save your changes.

You can now include the MathLibrary.h file and use the Functions class in your client application. Replace the contents of MathClient.cpp by using the following code:

// MathClient.cpp : Defines the entry point for the console application.  
// Compile by using: cl /EHsc /link MathLibrary.lib MathClient.cpp  

#include "stdafx.h"  
#include <iostream>  
#include "MathLibrary.h"  

using namespace std;  

int main()  
{  
    double a = 7.4;  
    int b = 99;  

    cout << "a + b = " <<  
        MathLibrary::Functions::Add(a, b) << endl;  
    cout << "a * b = " <<  
        MathLibrary::Functions::Multiply(a, b) << endl;  
    cout << "a + (a * b) = " <<  
        MathLibrary::Functions::AddMultiply(a, b) << endl;  

    return 0;  
}

Build the application by choosing Build, Build Solution on the menu bar. The Output window in Visual Studio might contain something like this:

1>------ Build started: Project: MathLibrary, Configuration: Debug Win32 ------  
1>  MathLibrary.cpp  
1>  MathLibrary.vcxproj -> c:\users\username\documents\visual studio 2015\Projects\MathLibraryAndClient\Debug\MathLibrary.dll  
2>------ Build started: Project: MathClient, Configuration: Debug Win32 ------  
2>  stdafx.cpp  
2>  MathClient.cpp  
2>  MathClient.vcxproj -> c:\users\username\documents\visual studio 2015\Projects\MathLibraryAndClient\Debug\MathClient.exe  
========== Build: 2 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========

Congratulations, you've created an application that calls functions in a DLL. Next, you'll run your application to see what it does.

To run the application

Since you can't run a DLL, make sure that MathClient is selected as the default project. This is the project that Visual Studio runs when you choose the Start Debugging or Start Without Debugging commands. In Solution Explorer, select MathClient, and then on the menu bar, choose Project, Set As StartUp Project.
To run the MathClient project, on the menu bar, choose Debug, Start Without Debugging. Visual Studio opens a command window for the program to run in. The output should resemble this:
```
a + b = 106.4  
a * b = 732.6  
a + (a * b) = 740  
Press any key to continue . . .  
```
You can press any key to dismiss the command window.

Now that you've created a DLL and a client application, you can experiment. Try setting breakpoints in the code of the client app or in the library, and run the app in the debugger. Add other members to the Functions class, or add a new class.

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