C invalidating

If this parameter is TRUE, the background is erased when the Begin Paint function is called.

If this parameter is FALSE, the background remains unchanged.

push_back will most likely trigger a resize for the vector, and that will invalidate references to elements in the vector. It again might seem wrong, but this in fact is valid C code.

num will end up pointing anywhere and so using it to call do_something is not valid C . Common sense might tell us that a call to push_back automatically invalidates references to elements in the vector, and it only works because most implementations will do the reasonable thing (ie not to invalidate references unless they must).

Turns out the standard makes a special prevision for this case in section 23.3.6.5: a resize for a vector is guaranteed to be triggerd if, and only if, the capacity of the vector is not enough, and references to elements in the vector are guaranteed to be valid unless resize is triggered.

A bit of language laweyering shows that what seems like an error is in fact allowed by the standard, but even if this is valid C code you should always keep in mind that assuming that the capacity of a vector will be enough is a VERY big assumption, it’s very easy to break and you won’t get any warning when it happens (maybe a core dump, if you’re lucky).

These functions mark all or part of a client area as invalid (that must be redrawn).

In the following example, the window procedure invalidates the entire client area when processing WM_CHAR messages.

For Date Time column, I want to use datetimepicker control to edit the value. https://msdn.microsoft.com/en-us/library/7tas5c80 After assigning the custom cell template to the Date Time column, datagridview is not invalidating immediately.

If the function succeeds, the return value is nonzero. The invalidated areas accumulate in the update region until the region is processed when the next WM_PAINT message occurs or until the region is validated by using the Validate Rect or Validate Rgn function.

The system sends a WM_PAINT message to a window whenever its update region is not empty and there are no other messages in the application queue for that window.

The following code example enables the user to drag an image or image file onto the form, and have it be displayed at the point on it is dropped.

The On Paint method is overridden to repaint the image each time the form is painted; otherwise the image would only persist until the next repainting.