QLibrary Class

Member Function Documentation

[explicit] QLibrary::QLibrary(QObject *parent = nullptr)

Constructs a library with the given parent.

[explicit] QLibrary::QLibrary(const QString &fileName, QObject *parent = nullptr)

Constructs a library object with the given parent that will load the library specified by fileName.

We recommend omitting the file's suffix in fileName, since QLibrary will automatically look for the file with the appropriate suffix in accordance with the platform, e.g. ".so" on Unix, ".dylib" on macOS and iOS, and ".dll" on Windows. (See fileName.)

[explicit] QLibrary::QLibrary(const QString &fileName, const QString &version, QObject *parent = nullptr)

Constructs a library object with the given parent that will load the library specified by fileName and full version number version. Currently, the version number is ignored on Windows.

We recommend omitting the file's suffix in fileName, since QLibrary will automatically look for the file with the appropriate suffix in accordance with the platform, e.g. ".so" on Unix, ".dylib" on macOS and iOS, and ".dll" on Windows. (See fileName.)

[explicit] QLibrary::QLibrary(const QString &fileName, int verNum, QObject *parent = nullptr)

Constructs a library object with the given parent that will load the library specified by fileName and major version number verNum. Currently, the version number is ignored on Windows.

We recommend omitting the file's suffix in fileName, since QLibrary will automatically look for the file with the appropriate suffix in accordance with the platform, e.g. ".so" on Unix, ".dylib" on macOS and iOS, and ".dll" on Windows. (See fileName.)

[virtual noexcept] QLibrary::~QLibrary()

Destroys the QLibrary object.

Unless unload() was called explicitly, the library stays in memory until the application terminates.

See also isLoaded() and unload().

QString QLibrary::errorString() const

Returns a text string with the description of the last error that occurred. Currently, errorString will only be set if load(), unload() or resolve() for some reason fails.

[static] bool QLibrary::isLibrary(const QString &fileName)

Returns true if fileName has a valid suffix for a loadable library; otherwise returns false.

Trailing versioning numbers on Unix are ignored.

bool QLibrary::isLoaded() const

Returns true if load() succeeded; otherwise returns false.

See also load().

bool QLibrary::load()

Loads the library and returns true if the library was loaded successfully; otherwise returns false. Since resolve() always calls this function before resolving any symbols it is not necessary to call it explicitly. In some situations you might want the library loaded in advance, in which case you would use this function.

See also unload().

QFunctionPointer QLibrary::resolve(const char *symbol)

Returns the address of the exported symbol symbol. The library is loaded if necessary. The function returns nullptr if the symbol could not be resolved or if the library could not be loaded.

Example:

typedef int (*AvgFunction)(int, int);

AvgFunction avg = (AvgFunction) library->resolve("avg");
if (avg)
    return avg(5, 8);
else
    return -1;

The symbol must be exported as a C function from the library. This means that the function must be wrapped in an extern "C" if the library is compiled with a C++ compiler. On Windows you must also explicitly export the function from the DLL using the __declspec(dllexport) compiler directive, for example:

extern "C" MY_EXPORT int avg(int a, int b)
{
    return (a + b) / 2;
}

with MY_EXPORT defined as

#ifdef Q_OS_WIN
#define MY_EXPORT __declspec(dllexport)
#else
#define MY_EXPORT
#endif

[static] QFunctionPointer QLibrary::resolve(const QString &fileName, const char *symbol)

This is an overloaded function.

Loads the library fileName and returns the address of the exported symbol symbol. Note that fileName should not include the platform-specific file suffix; (see fileName). The library remains loaded until the application exits.

The function returns nullptr if the symbol could not be resolved or if the library could not be loaded.

See also resolve().

[static] QFunctionPointer QLibrary::resolve(const QString &fileName, const QString &version, const char *symbol)

This is an overloaded function.

Loads the library fileName with full version number version and returns the address of the exported symbol symbol. Note that fileName should not include the platform-specific file suffix; (see fileName). The library remains loaded until the application exits. version is ignored on Windows.

The function returns nullptr if the symbol could not be resolved or if the library could not be loaded.

See also resolve().

[static] QFunctionPointer QLibrary::resolve(const QString &fileName, int verNum, const char *symbol)

This is an overloaded function.

Loads the library fileName with major version number verNum and returns the address of the exported symbol symbol. Note that fileName should not include the platform-specific file suffix; (see fileName). The library remains loaded until the application exits. verNum is ignored on Windows.

The function returns nullptr if the symbol could not be resolved or if the library could not be loaded.

See also resolve().

void QLibrary::setFileNameAndVersion(const QString &fileName, const QString &version)

Sets the fileName property and full version number to fileName and version respectively. The version parameter is ignored on Windows.

See also setFileName().

void QLibrary::setFileNameAndVersion(const QString &fileName, int versionNumber)

Sets the fileName property and major version number to fileName and versionNumber respectively. The versionNumber is ignored on Windows.

See also setFileName().

bool QLibrary::unload()

Unloads the library and returns true if the library could be unloaded; otherwise returns false.

This happens automatically on application termination, so you shouldn't normally need to call this function.

If other instances of QLibrary are using the same library, the call will fail, and unloading will only happen when every instance has called unload().

Note that on macOS, dynamic libraries cannot be unloaded. QLibrary::unload() will return true, but the library will remain loaded into the process.

See also resolve() and load().