Ensure that the framebuffer object is properly created before using it.

src/Magnum/AbstractFramebuffer.cpp

@@ -82,6 +82,17 @@ Int AbstractFramebuffer::maxDualSourceDrawBuffers() {
 }
 #endif
 
+void AbstractFramebuffer::createIfNotAlready() {
+    if(_created) return;
+
+    /* glGen*() does not create the object, just reserves the name. Some
+       commands (such as glObjectLabel()) operate with IDs directly and they
+       require the object to be created. Binding the framebuffer finally
+       creates it. Also all EXT DSA functions implicitly create it. */
+    bindInternal();
+    CORRADE_INTERNAL_ASSERT(_created);
+}
+
 void AbstractFramebuffer::bind(FramebufferTarget target) {
     bindInternal(target);
     setViewportInternal();
@@ -102,6 +113,8 @@ void AbstractFramebuffer::bindInternal(FramebufferTarget target) {
         state->readBinding = state->drawBinding = _id;
     } else CORRADE_ASSERT_UNREACHABLE();
 
+    /* Binding the framebuffer finally creates it */
+    _created = true;
     glBindFramebuffer(GLenum(target), _id);
 }
 
@@ -119,6 +132,8 @@ FramebufferTarget AbstractFramebuffer::bindInternal() {
     /* Or bind it, if not already */
     state->readBinding = _id;
 
+    /* Binding the framebuffer finally creates it */
+    _created = true;
     #ifndef MAGNUM_TARGET_GLES2
     glBindFramebuffer(GLenum(FramebufferTarget::Read), _id);
     return FramebufferTarget::Read;
@@ -256,6 +271,7 @@ GLenum AbstractFramebuffer::checkStatusImplementationDefault(const FramebufferTa
 
 #ifndef MAGNUM_TARGET_GLES
 GLenum AbstractFramebuffer::checkStatusImplementationDSA(const FramebufferTarget target) {
+    _created = true;
     return glCheckNamedFramebufferStatusEXT(_id, GLenum(target));
 }
 #endif
@@ -280,6 +296,7 @@ void AbstractFramebuffer::drawBuffersImplementationDefault(GLsizei count, const
 
 #ifndef MAGNUM_TARGET_GLES
 void AbstractFramebuffer::drawBuffersImplementationDSA(GLsizei count, const GLenum* buffers) {
+    _created = true;
     glFramebufferDrawBuffersEXT(_id, count, buffers);
 }
 #endif
@@ -305,6 +322,7 @@ void AbstractFramebuffer::drawBufferImplementationDefault(GLenum buffer) {
 
 #ifndef MAGNUM_TARGET_GLES
 void AbstractFramebuffer::drawBufferImplementationDSA(GLenum buffer) {
+    _created = true;
     glFramebufferDrawBufferEXT(_id, buffer);
 }
 #endif
@@ -328,6 +346,7 @@ void AbstractFramebuffer::readBufferImplementationDefault(GLenum buffer) {
 
 #ifndef MAGNUM_TARGET_GLES
 void AbstractFramebuffer::readBufferImplementationDSA(GLenum buffer) {
+    _created = true;
     glFramebufferReadBufferEXT(_id, buffer);
 }
 #endif

src/Magnum/AbstractFramebuffer.h

@@ -329,11 +329,14 @@ class MAGNUM_EXPORT AbstractFramebuffer {
         explicit AbstractFramebuffer() = default;
         ~AbstractFramebuffer() = default;
 
+        void MAGNUM_LOCAL createIfNotAlready();
+
         void MAGNUM_LOCAL bindInternal(FramebufferTarget target);
         FramebufferTarget MAGNUM_LOCAL bindInternal();
         void MAGNUM_LOCAL setViewportInternal();
 
         GLuint _id;
+        bool _created; /* see createIfNotAlready() for details */
         Range2Di _viewport;
 
     private:

src/Magnum/DefaultFramebuffer.cpp

@@ -36,7 +36,10 @@ namespace Magnum {
 
 DefaultFramebuffer defaultFramebuffer;
 
-DefaultFramebuffer::DefaultFramebuffer() { _id = 0; }
+DefaultFramebuffer::DefaultFramebuffer() {
+    _id = 0;
+    _created = true;
+}
 
 DefaultFramebuffer::Status DefaultFramebuffer::checkStatus(const FramebufferTarget target) {
     return Status((this->*Context::current()->state().framebuffer->checkStatusImplementation)(target));

src/Magnum/Framebuffer.cpp

@@ -81,6 +81,7 @@ Int Framebuffer::maxColorAttachments() {
 
 Framebuffer::Framebuffer(const Range2Di& viewport) {
     _viewport = viewport;
+    _created = false;
 
     glGenFramebuffers(1, &_id);
     CORRADE_INTERNAL_ASSERT(_id != Implementation::State::DisengagedBinding);
@@ -108,11 +109,13 @@ Framebuffer::~Framebuffer() {
     glDeleteFramebuffers(1, &_id);
 }
 
-std::string Framebuffer::label() const {
+std::string Framebuffer::label() {
+    createIfNotAlready();
     return Context::current()->state().debug->getLabelImplementation(GL_FRAMEBUFFER, _id);
 }
 
 Framebuffer& Framebuffer::setLabelInternal(const Containers::ArrayReference<const char> label) {
+    createIfNotAlready();
     Context::current()->state().debug->labelImplementation(GL_FRAMEBUFFER, _id, label);
     return *this;
 }
@@ -255,6 +258,7 @@ void Framebuffer::renderbufferImplementationDefault(BufferAttachment attachment,
 
 #ifndef MAGNUM_TARGET_GLES
 void Framebuffer::renderbufferImplementationDSA(BufferAttachment attachment, Renderbuffer& renderbuffer) {
+    _created = true;
     glNamedFramebufferRenderbufferEXT(_id, GLenum(attachment), GL_RENDERBUFFER, renderbuffer.id());
 }
 
@@ -263,6 +267,7 @@ void Framebuffer::texture1DImplementationDefault(BufferAttachment attachment, GL
 }
 
 void Framebuffer::texture1DImplementationDSA(BufferAttachment attachment, GLuint textureId, GLint mipLevel) {
+    _created = true;
     glNamedFramebufferTexture1DEXT(_id, GLenum(attachment), GL_TEXTURE_1D, textureId, mipLevel);
 }
 #endif
@@ -273,6 +278,7 @@ void Framebuffer::texture2DImplementationDefault(BufferAttachment attachment, GL
 
 #ifndef MAGNUM_TARGET_GLES
 void Framebuffer::texture2DImplementationDSA(BufferAttachment attachment, GLenum textureTarget, GLuint textureId, GLint mipLevel) {
+    _created = true;
     glNamedFramebufferTexture2DEXT(_id, GLenum(attachment), textureTarget, textureId, mipLevel);
 }
 #endif
@@ -293,6 +299,7 @@ void Framebuffer::textureLayerImplementationDefault(BufferAttachment attachment,
 
 #ifndef MAGNUM_TARGET_GLES
 void Framebuffer::textureLayerImplementationDSA(BufferAttachment attachment, GLuint textureId, GLint mipLevel, GLint layer) {
+    _created = true;
     glNamedFramebufferTextureLayerEXT(_id, GLenum(attachment), textureId, mipLevel, layer);
 }
 #endif

src/Magnum/Framebuffer.h

@@ -341,7 +341,7 @@ class MAGNUM_EXPORT Framebuffer: public AbstractFramebuffer, public AbstractObje
          *      @fn_gl_extension2{GetObjectLabel,EXT,debug_label} with
          *      @def_gl{FRAMEBUFFER}
          */
-        std::string label() const;
+        std::string label();
 
         /**
          * @brief Set framebuffer label
@@ -664,6 +664,7 @@ Debug MAGNUM_EXPORT operator<<(Debug debug, Framebuffer::Status value);
 inline Framebuffer::Framebuffer(Framebuffer&& other) noexcept {
     _id = other._id;
     _viewport = other._viewport;
+    _created = other._created;
     other._id = 0;
     other._viewport = {};
 }
@@ -671,6 +672,7 @@ inline Framebuffer::Framebuffer(Framebuffer&& other) noexcept {
 inline Framebuffer& Framebuffer::operator=(Framebuffer&& other) noexcept {
     std::swap(_id, other._id);
     std::swap(_viewport, other._viewport);
+    std::swap(_created, other._created);
     return *this;
 }
 

src/Magnum/Test/FramebufferGLTest.cpp

@@ -215,19 +215,7 @@ void FramebufferGLTest::label() {
        !Context::current()->isExtensionSupported<Extensions::GL::EXT::debug_label>())
         CORRADE_SKIP("Required extension is not available");
 
-    {
-        /** @todo Is this even legal optimization? */
-        CORRADE_EXPECT_FAIL("The object must be used at least once before setting/querying label.");
-        CORRADE_VERIFY(false);
-    }
-    Renderbuffer renderbuffer;
-    #ifndef MAGNUM_TARGET_GLES2
-    renderbuffer.setStorage(RenderbufferFormat::RGBA8, {128, 128});
-    #else
-    renderbuffer.setStorage(RenderbufferFormat::RGBA4, {128, 128});
-    #endif
     Framebuffer framebuffer({{}, Vector2i(32)});
-    framebuffer.attachRenderbuffer(Framebuffer::BufferAttachment(Framebuffer::BufferAttachment::Depth), renderbuffer);
 
     CORRADE_COMPARE(framebuffer.label(), "");
     MAGNUM_VERIFY_NO_ERROR();