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inactiveTopic C++ Expression Templates
started 3/9/2004; 3:22:16 AM - last post 3/9/2004; 11:39:13 AM
Ehud Lamm - C++ Expression Templates  blueArrow
3/9/2004; 3:22:16 AM (reads: 7105, responses: 2)
C++ Expression Templates
Another C++ template metaprogramming article from Todd Veldhuizen.

Seeing as expression templates are used as a C++ technique for passing expressions as function arguments, even functional programming fanatics should agree that knowing this technique is a step in the right direction.

Posted to Meta Programming by Ehud Lamm on 3/9/04; 3:22:39 AM

Dan Shappir - Re: C++ Expression Templates  blueArrow
3/9/2004; 8:06:49 AM (reads: 248, responses: 1)
Back in 97, this article and another article by Todd published at DDJ, inspired me to write my own article at WDJ. In this article I applied the same technique to improve the performance of string concatenation operations. Consider the following code snippet (which uses CString, the MFC string class, but would hold for std::string as well):

CString a = "hello ";
CString b = "there ";
CString c = "world";
CString d = a + b + c;

This code actually translates to:

CString d = CString(a + b) + c;

This means a temporary object is created a destroyed and the contents of a and d are copied twice. Using template metaprogramming the expression a + b + c actually yields an object that represents the expression - a parse tree. A new constructor accepts this object type and traverses it twice: once to calculate to total length and a second time to perform the actual copy.

The result of this modification was a significant performance boost, without the class user needing to modify his code.

Ehud Lamm - Re: C++ Expression Templates  blueArrow
3/9/2004; 11:39:13 AM (reads: 205, responses: 0)