The size of cheerp output JavaScript code is very large. Is this normal? What can I do to reduce it?

If you only use browser APIs in your code and no functions from the C or C++ library you can compile using the -nostdlib option. This will reduce the code size by not linking in initialization code for <iostream> functionalities. Please node that Cheerp already compiles only the code which is actually used, but currently the static initializations of std::cout and similar objects are seen as executed code.

Does Cheerp enforce C++ type safety at run-time?

No. Similarly to what happens when compiling native code from C++, type information is only used to enforce type safety at compile time. Generated code (both from cheerp or from traditional compilers) keeps no type information at run-time. This also means that type safety has no overhead at run-time. If you change the internal representation of an object using e.g. a JavaScript debugger, the results will probably be bad, unless you know very well what you are doing.

What features of C++ are currently not supported?

cheerp currently supports most of C++ features. While we try very hard to have a full C++14 compatibility, some features are currently not supported:

  • Global ordering of pointers. This means that pointers have a definite ordering (as required by the standard) only if they are part of the same array. Unrelated pointers at the moment have undefined ordering, which means that constructs like std::map<void*> will not work in cheerp.
  • Exceptions. This limitation is caused by the fact that JavaScript engines handle JavaScript exceptions very poorly. There are ways to workaround this limitation and exceptions will be supported in the future. To simplify the porting of existing codebases you can pass the -fexceptions command line to Cheerp, the compiler will correctly parse and ignore exception handlers. If an exception is actually thrown at runtime an abort will be triggered.

Is cheerp free software? Or is it proprietary?

Cheerp is 100% free software. The cheerp compiler itself is licensed under the University of Illinois/NCSA license, like the LLVM and clang projects the cheerp compiler is based on. Cheerp headers and libraries which provide access to browser and other functionalities both at low and high level are released under the GPLv2+ license.

Should I release my code as GPL if I use cheerp?

There are two cases:

  • If you don’t want to release the source code of your programs, Leaning Technologies Ltd (the company behind the development of Cheerp) offers proprietary-friendly commercial licenses and support. Please visit for more infomation.
  • If you are using our free GPL version you should release the code. You are using GPL headers and linking with GPL libraries so your code is considered derived work of cheerp.