Disable instantiation of all Abstract* classes.

For classes which already have pure virtual functions instantiation is
not allowed, but for other there needs to be at least one pure virtual
method: the destructor.

Pure virtual functions actually can have implementations, but they must
be called explicitly. Destructors are called explicitly, so for them it
works.

src/AbstractImage.h

@@ -216,7 +216,7 @@ class MAGNUM_EXPORT AbstractImage {
         inline AbstractImage(Components components, ComponentType type): _components(components), _type(type) {}
 
         /** @brief Destructor */
-        inline virtual ~AbstractImage() {}
+        virtual ~AbstractImage() = 0;
 
         /** @brief Color components */
         inline Components components() const { return _components; }
@@ -229,6 +229,8 @@ class MAGNUM_EXPORT AbstractImage {
         ComponentType _type;        /**< @brief Data type */
 };
 
+inline AbstractImage::~AbstractImage() {}
+
 }
 
 #endif

src/AbstractShaderProgram.h

@@ -121,7 +121,7 @@ class MAGNUM_EXPORT AbstractShaderProgram {
          *
          * Deletes associated OpenGL shader program.
          */
-        inline ~AbstractShaderProgram() { glDeleteProgram(program); }
+        virtual ~AbstractShaderProgram() = 0;
 
         /**
          * @brief Use shader
@@ -300,6 +300,8 @@ class MAGNUM_EXPORT AbstractShaderProgram {
         State state;
 };
 
+inline AbstractShaderProgram::~AbstractShaderProgram() { glDeleteProgram(program); }
+
 }
 
 #endif

src/AbstractTexture.h

@@ -472,9 +472,7 @@ class MAGNUM_EXPORT AbstractTexture {
          *
          * Deletes assigned OpenGL texture.
          */
-        inline virtual ~AbstractTexture() {
-            glDeleteTextures(1, &texture);
-        }
+        virtual ~AbstractTexture() = 0;
 
         /** @brief %Texture layer */
         inline GLint layer() const { return _layer; }
@@ -568,6 +566,8 @@ class MAGNUM_EXPORT AbstractTexture {
         GLuint texture;
 };
 
+inline AbstractTexture::~AbstractTexture() { glDeleteTextures(1, &texture); }
+
 /** @relates AbstractTexture
 @brief Convertor of component count and data type to InternalFormat
 */