Compiling header using C, instead of C++ -

i have c++ project have configured using cmake use eclipse. problem have added static c library (namely, svm-struct/svm-light) doesn't seem compile- , guess it's compiling c++ instead of c.

i added library project follows:

set(svm_light_src_dir "../../libraries/svm_rank") include_directories(${svm_light_src_dir})  add_library(     svm_rank_lib static     ${svm_light_src_dir}/svm_light/svm_learn.c     ${svm_light_src_dir}/svm_light/svm_common.c     ${svm_light_src_dir}/svm_light/svm_hideo.c     ${svm_light_src_dir}/svm_struct/svm_struct_learn.c     ${svm_light_src_dir}/svm_struct/svm_struct_common.c     ${svm_light_src_dir}/svm_struct/svm_struct_classify.c     ${svm_light_src_dir}/svm_struct_api.c     ${svm_light_src_dir}/svm_struct_learn_custom.c )  add_executable(${project_name} ${sources}) target_link_libraries(${project_name} svm_rank_lib) 

cmake configures fine seems. within output of configuration specifies finds c , c++ compilers , "work". add header 1 of project files using extern follows:

#ifdef __cplusplus extern "c" { # include "svm_struct/svm_struct_common.h" } #endif 

when go build project error here:

../../libraries/svm_rank/svm_struct/../svm_struct_api_types.h:75:11: error: expected member name or ';' after declaration specifiers   double *class;    /* vector of scores imply ranking */   ~~~~~~  ^ 1 error generated. 

there variable within library header called "class" error occurs, , guess it's trying compile library header using c++ instead of c. first of reason error? if so, how should go fixing this?

as has been pointed out, source of problem c library header declares variable named class, keyword in c++.

this problem hit header pulled in c++ source file. remember headers not compiled themselves, merely copy-pasted preprocessor source file #includes them. type of source file determines whether code in header interpreted c or c++.

the fact wrapped include in extern "c" not change this. switches off c++-style name mangling declarations in header, code still has compile valid c++.

the cleanest solution problem technique known insulation or compiler firewall.

you have make sure parts come contact problematic library c-source files themselves. c++ part of code interacts library through interface of c part, never library directly. in particular, must never #include library headers of header files.

for instance: my_interface.c

#include "svm_struct/svm_struct_common.h"  /* safe include .c file */  struct opaque_ {      /* can use types svm_struct_common in here */ };  opaque* initialize() {      /* can create opaque_ on heap ,         manipulate here, give         pointer c++ part */ }  void do_stuff(opaque*) {     /* whatever stuff in opaque */ } 

my_interface.h

/* no #includes in header! */  /* opaque type forward declared!    c++ code can obtain pointer it,    cannot inside */ struct opaque_; typedef struct opaque_ opaque;  opaque* initialize(); void do_stuff(opaque*); 

my_application.cpp

// include our own header, made sure valid c++ extern "c" {     #include <my_interface.h> }  void do_stuff() {     opaque* context = initialize();     do_stuff(context); }