CODEWITHSUNDEEP
c++boostlinkerinclude
4ae1e1
4ae1e1

Reputation: 7634

C++: get Boost working; questions on include path and linking library

I was trying to use Boost.Tokenizer library. In my PROG.cpp, I have the following:

#include <boost/tokenizer.hpp>

And my Makefile was initially something like

CXX = g++-4.8
CXXFLAGS = ## some irrelevant flags
LDFLAGS = ## some irrelevant flags
SOURCES = PROG.cpp
OBJECTS = $(SOURCES:.cpp=.o)
TARGETS = PROG

$(TARGETS) : $(OBJECTS)
        $(CXX) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)

## other targets

It won't compile, since boost/tokenizer.hpp cannot be found:

fatal error: boost/tokenizer.hpp: No such file or directory

Then I manually added the boost include path to CXXFLAGS:

-I/opt/local/include/

(which is the path from MacPorts.)

Then I tried to include the Tokenizer library, but in /opt/local/lib/ I have libboost_atomic-mt.dylib, libboost_chrono-mt.dylib, etc., but nothing like tokenizer. I was rather confused at the time. I supposed that still wouldn't work since the library was not linked against. Surprisingly, the program built, linked, and ran perfectly.

So I'm really confused now. Here are some questions:

(1) I did not link against boost explicitly, so boost is treated like standard library by the linker?

(2) If boost is treated like standard, why the headers are not standard?

(3) Why there are libboost_atomic-mt.dylib, libboost_chrono-mt.dylib, etc. but not tokenizer? Which dynamic library does tokenizer belong to?

I'm not very familiar with g++ linking mechanism; speaking of boost, this is my very first program with boost. So I'd really appreciate detailed explanation. Thanks in advance!

For reference, this is what I extracted by gcc -print-search-dirs:

install: /usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/
programs: =/usr/gcc-4.8.0/libexec/gcc/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/libexec/gcc/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/libexec/gcc/x86_64-apple-darwin12.3.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../../x86_64-apple-darwin12.3.0/bin/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../../x86_64-apple-darwin12.3.0/bin/
libraries: =/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../../x86_64-apple-darwin12.3.0/lib/x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../../x86_64-apple-darwin12.3.0/lib/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../x86_64-apple-darwin12.3.0/4.8.0/:/usr/gcc-4.8.0/lib/gcc/x86_64-apple-darwin12.3.0/4.8.0/../../../:/lib/x86_64-apple-darwin12.3.0/4.8.0/:/lib/:/usr/lib/x86_64-apple-darwin12.3.0/4.8.0/:/usr/lib/

Upvotes: 2

Views: 15187

Answers (1)

Salgar
Salgar

Reputation: 7775

Most of the boost libraries, are just header files, if you look in the .hpp files you will not see just the declaration of the classes, like you would expect in a header file, but actually the entire implementation. This is why for 90% of the boost libraries, you don't need to worry about linking, only inclusion.

However for a few libraries, serialiser, a few others, there is just too much polluting code for the header inclusion method to be reasonable. I'm sure there is a better, more rigid definition about when the implementation is included in the header and when it isn't.

http://www.boost.org/boost-build2/doc/html/bbv2/faq/header-only-libraries.html

Here is another question about it: Why are not all boost libraries header-only?

p.s. Generally it is better to keep the boost library separate and in your makefile do something like:

For compilation: CXXFLAGS += -I/path/to/boost/include

For Linking: LDPATH += -L/path/to/boost/lib

This makes it easier to upgrade your boost version as you just have to change the path in one place.

Upvotes: 7

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