How To Use ICU

ICU builds and installs as relatively standard libraries. For details about building, installing and porting see the ICU4C readme and the ICU4J readme. In addition, ICU4C installs several scripts and makefile fragments that help build other code using ICU.

For C++, note that there are Recommended Build Options (both for normal use and for ICU as system-level libraries) which are not default simply for compatibility with older ICU-using code.

Starting with ICU 49, the ICU4C readme has a short section about User-Configurable Settings.

C++ Makefiles

The recommended way to use ICU in Makefiles is to use the pkg-config files which are installed by ICU upon "make install". There are   files for various libraries and components. This is preferred over the deprecated icu-config script. Below are the package names used within pkg-config.

Package Contents
icu-uc Common (uc) and Data (dt/data) libraries
icu-i18n Internationalization (in/i18n) library
icu-le Layout Engine
icu-lx Paragraph Layout
icu-io Ustdio/iostream library (icuio)

For example, to compile a simple application, you could run the following command. See the pkg-config manpage for more details.

    c++ -o test test.c `pkg-config --libs --cflags icu-uc icu-io`

ICU installs the pkg-config (.pc) files in $(prefix)/lib/pkgconfig (where $(prefix) is the installation prefix for ICU). It may be necessary to add $(prefix)/lib/pkgconfig to the PKG_CONFIG_PATH variable.

If you use ICU in a small project, it may be convenient to take advantage of ICU's autoconf'ed files. ICU make install writes $(prefix)/lib/icu/ which defines most of the necessary make variables like $(CXX), $(CXXFLAGS), $(ICULIBS), $(INVOKE), $(ICUPKG), $(datadir), etc. By itself, is incomplete: It assumes that it is included into another Makefile which also defines $(srcdir), $(DYNAMICCXXFLAGS) and similar. It is probably best to copy ICU's autoconf'ed top-level ./Makefile and/or library-style i18n/Makefile and/or binary-style tools/icupkg/Makefile and modify them as needed.

If you use ICU in a medium-size project where you use your own autoconf/CMake/... setup, you will probably want to cherry-pick the definitions you need, for example paths to ICU data and tools, rather than taking the entire and overriding definitions that are different for your project. For selective ICU definitions, use the installed $(prefix)/bin/icu-config script whose contents are synchronized with $(prefix)/lib/icu/ For example, use `icu-config --invoke=icupkg` to invoke the ICU .dat packaging tool.

If you use ICU in a large project, you probably have your own build system and will just use ICU's public header files, .so files, etc. See the next section, "C++ With Your Own Build System"

Note icu-config is deprecated, and no longer recommended for production use. Please use pkg-config files or other options. 

As of ICU 63.1, icu-config has been deprecated (ICU-10464). icu-config be disabled by default in the future. As of ICU 63.1 enable or disable 63.1 with: --enable-icu-config or --disable-icu-config

icu-config is installed (by ICU's make install) into  $(prefix)/bin/icu-config and can be convenient for trivial, single-file programs using ICU. For example, you could compile and build a small program with `icu-config --cxx --cxxflags --cppflags --ldflags` -o sample sample.cpp. The icu-config script comes with a good man page.

C++ With Your Own Build System

If you are not using the standard build system, you will need to construct your own system. Here are a couple of starting points:
  • At least for initial bring-up, use pre-built data files from the ICU download or from a normally-built ICU. Copy the icudtXXx.dat file from icu/source/data/in/ or icu/source/data/out/tmp/ in either of these two locations, into icu/source/data/in/ on your target ICU system. That way, you won't need to build ICU's data-generation tools.
  • Don't compile all files. Look in the files for OBJECTS= clauses which will indicate which source files should be compiled. (Some .c files are #included into others and cannot be compiled by themselves.)
  • ICU does not throw or handle exceptions. Consider turning them off via g++'s -fno-exceptions or equivalent.
  • Each ICU library needs to be compiled with -DU_COMMON_IMPLEMENTATION, -DU_I18N_IMPLEMENTATION etc. as appropriate. See unicode/utypes.h for the set of such macros. If you build one single DLL (shared library) for all of ICU, also use -DU_COMBINED_IMPLEMENTATION. If you build ICU as statically-linked libraries, use -DU_STATIC_IMPLEMENTATION.
  • Use the icu-support mailing list. Ask for help and guidance on your strategy.
  • Up until ICU 4.8, there are one or two header files (platform.h, icucfg.h) that are generated by autoconf/configure and thus differ by platform, sometimes even by target settings on a single platform (e.g., AIX 32-bit vs. 64-bit, Mac OS X universal binaries PowerPC vs. x86). If you do not use autoconf, you probably need to configure-generate these header files for your target platforms and select among them, or merge the generated headers if they are similar, or simulate their generation by other means.
  • Starting with ICU 49, all source code files are fixed (not generated). In particular, there is one single platform.h file which determines platform-specific settings via #if ...

C++ Namespace

ICU C++ APIs are normally defined in a versioned namespace, for example "icu_50". There is a stable "icu" alias which should be used instead. (Entry point versioning is only to allow for multiple ICU versions linked into one program. It is optional and should be off for system libraries.)

By default, and only for backward compatibility, the ICU headers contain a line using namespace icu_50; which makes all ICU APIs visible in/with the global namespace (and potentially collide with non-ICU APIs there). One of the Recommended Build Options is to turn this off.

To write forward declarations, use

class UnicodeSet;
class UnicodeString;


To qualify an ICU class name, use the "icu" alias:

static myFunction(const icu::UnicodeString &s) {...}

Frequently used ICU classes can be made easier to use in .cpp files with

using icu::UnicodeSet;
using icu::UnicodeString;

Other Notes

Helper Install Utilities

ICU installs $(prefix)/share/icu/$(VERSION)/install-sh and $(prefix)/share/icu/$(VERSION)/mkinstalldirs which may be used by ICU tools and samples. Their paths are given in the installed  ( see above).

Data Packaging Settings

The pkgdata  tool (see Packaging ICU4C ) makes use of the installed file $(prefix)/lib/icu/ to set parameters for data packaging operations that require use of platform compilers and linkers ( in static or dll mode).  pkgdata uses the icu-config script in order to locate If you are not building ICU using the supplied tools, you may need to modify this file directly to allow static and dll modes to function.

Building and Running Trivial C/C++ Programs with icurun

For building and running trivial (one-compilation-unit) programs with an installed ICU4C, the shell script icurun may be used. For detailed help, see the top of that script.
As an example, if ICU is installed to the prefix /opt/local  and the current directory contains two sample programs "test1.cpp" and "test2.c", they may be compiled and run with any of the following commands.  The "-i" option specifies either the installed icu-config script,  or the directory containing that script, or the path to a 'bin' directory.
  • icurun -i /opt/local                 test1.cpp
  • icurun -i /opt/local/bin             test2.c
  • icurun -i /opt/local/bin/icu-config  test1.cpp
If "icu-config" is on the PATH, the -i option may be omitted:
  • icurun  test1.cpp
Any additional arguments will be passed to the program.
  • icurun  test1.cpp  args...
This feature is a work in progress, please give feedback at Ticket #8481