Why does using boost increase file size so much?

I've noticed that when I use a boost feature the app size tends to increase by about .1 - .3 MB. This may not seem like much, but compared to using other external libraries i开发者_开发问答t is (for me at least). Why is this?

Boost uses templates everywhere. These templates can be instantiated multiple times with the same parameters. A sufficiently smart linker will throw out all but one copy. However, not all linkers are sufficiently smart. Also, templates are instantiated implicitly sometimes and it's hard to even know how many times one has been instantiated.

"so much" is a comparative term, and I'm afraid you're comparing apples to oranges. Just because other libraries are smaller doesn't imply you should assume Boost is as small. Look at the sheer amount of work Boost does for you!

I doubt making a custom library with the same functionality would be of any considerable lesser size. The only valid comparison to make is "Boost's library that does X" versus "Another library that does X". Not "Boost's library that does X" and "Another library that does Y."

The file system library is very powerful, and this means lots of functions, and lot's of back-bone code to provide you and I with a simple interface. Also, like others mentioned templates in general can increase code size, but it's not like that's an avoidable thing. Templates or hand-coded, either one will results in the same size code. The only difference is templates are much easier.

It all depends on how it is used. Since Boost is a bunch of templates, it causes a bunch of member functions to be compiled per type used. If you use boost with n types, the member functions are defined (by C++ templates) n times, one for each type.

Boost consists primarily of very generalized and sometimes quite complex templates, which means, types and functions are created by the compiler as required per usage, and not simply by declaration. In other words, a small amount of source code can produce a significant quantity of object code to fulfill all variations of templates declared or used. Boost also depends on standard libraries, pulling in those dependencies as well. However, the most significant contribution is the fact that Boost source code exists almost primarily in include files. Including standard c include files (outside of STL) typically includes very little source code and contains mostly prototypes, small macros or type declarations without their implementations. Boost contains most of its implementations in its include file.