CString misuse #1

This is the first of many examples of ways to misuse and/or abuse MFC’s CString class. While this example (and following ones) are specific to MFC, they likely apply to all string classes (mutable or not). Here is the offending code:

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CString str( "First Part Of Message\n" );
str = str + "Second Part Of Message\n";
str = str + "Third Part Of Message";

MessageBox( str );
/////////////////////

If you write code like this, stop now and back slowly away from the keyboard – You’re Doing It Wrong!

First, the developer is adding (concatenating) strings together, but these are static/constant strings! They always add up to the same string, and as such can be made into a single constant string:

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"First Part Of Message\nSecond Part Of Message\nThird Part Of Message"
/////////////////////

So at a minimum, the start of the code should read:

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CString str( "First Part Of Message\nSecond Part Of Message\nThird Part Of Message" );
/////////////////////

Why not add up the strings separately like the original code did? Two reasons – overhead and exception opportunity. Each use of CString::operator+(…) can result in dynamic memory operations (allocation and deallocation). So you are looking at six potential heap operations (three potential allocations and deallocations including destruction, although in release builds of CString, the number of operations is less). Each operation has the potential to raise an exception and in the absence of per-thread heaps, can effectively bottleneck a multi-threaded application to the performance of a single-threaded one because the heap operations have to be serialized.

So by manually putting the strings together we have reduced heap operations from 6 to 2 – one allocation and one deallocation. That is a pretty good improvement, but we can do better!

The MessageBox(…) function does not take CStrings, it takes pointers to constant strings (LPCTSTR). So why is a CString needed here at all?

/////////////////////
MessageBox( "First Part Of Message\nSecond Part Of Message\nThird Part Of Message" );
/////////////////////

This final version of the code is simpler, will execute faster, and is more robust. Sounds like a winner to me!

Note: Some of you may be thinking about the preprocessor’s ability to automatically concatenate static strings. Yes it does, but it cannot automatically coalesce the above strings because they are separate – they are being passed (separately) as parameters to a function. If the + operator was not present in-between the parts of the string, they would be coalesced to a single string, but the unnecessary CString would still be there.

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