extern

问题 Is there any reasons for 'symbol multiply defined' other than not having the declaration in .h, having it as 'extern', and have the implementation in .cpp? I'm pretty sure that all my files follow the rule, but I'm getting an error message like this: ld: lto: could not merge in /Users/zlw/Library/Developer/Xcode/DerivedData/Wireless - amjmgyrircjezdhegioctszbcypz/Build/Intermediates/Wireless.build/Debug/Wireless.build/Objects normal/x86_64/qam.o because 'Linking globals named '

问题 I am exporting a method that can be called from unmanaged code, but the function itself lives in a managed c++ project. The following code results in a compiler error: error C2526: 'System::Collections::Generic::IEnumerator<T>::Current::get' : C linkage function cannot return C++ class 'System::Collections::Generic::KeyValuePair<TKey,TValue>' error C2526: 'System::Collections::Generic::Dictionary<TKey,TValue>::KeyCollection::GetEnumerator' : C linkage function cannot return C++ class 'System:

问题 I am exporting a method that can be called from unmanaged code, but the function itself lives in a managed c++ project. The following code results in a compiler error: error C2526: 'System::Collections::Generic::IEnumerator<T>::Current::get' : C linkage function cannot return C++ class 'System::Collections::Generic::KeyValuePair<TKey,TValue>' error C2526: 'System::Collections::Generic::Dictionary<TKey,TValue>::KeyCollection::GetEnumerator' : C linkage function cannot return C++ class 'System:

问题 There are two cases where typedef confuses me when it comes to extern template declaration and explicit template instantiation . To illustrate the two see below 2 example code snippets. Consider following example (Case 1) : // suppose following code in some cpp file template <typename T> struct example { T value; }; // valid typedefs typedef example<int> int_example; typedef example<std::string> string_example; // explicit instantiation using above typedefs template class int_example; // ->

问题 This question already has answers here : How do I use extern to share variables between source files? (17 answers) Can local and register variables be declared extern? (5 answers) Closed 2 years ago . Why does the following piece of code work - extern int i; main() { int i = 10; printf("%d", i); } but this one doesn't - main() { extern int i; int i = 10; printf("%d", i); } 回答1: As already stated in comments, in the first fragment there are two distinct variables; scoping rules make that the

问题 This question already has answers here : How do I use extern to share variables between source files? (17 answers) Can local and register variables be declared extern? (5 answers) Closed 2 years ago . Why does the following piece of code work - extern int i; main() { int i = 10; printf("%d", i); } but this one doesn't - main() { extern int i; int i = 10; printf("%d", i); } 回答1: As already stated in comments, in the first fragment there are two distinct variables; scoping rules make that the

问题 I try to declare a global variable config : //general.h struct config_t { int num; }; extern struct config_t config; //The global variable Then I define config variable in general.c : //general.c #include "general.h" struct config_t config = { num = 5; }; But, when I try to use the global variable 'config' in my main function, I get the error: undefined reference to `config': Main program: //main.c #include "general.h" main() { config.num = 10; } Why is it? 回答1: This looks like a linker error

问题 There are sources (books, online materials) that explain the usage of extern as following: extern int i; // declaration - has 'extern' int i = 1; // definition - specified by the absence of 'extern' And there are sources that support the following syntax: extern int i; // declaration extern int i = 1; // definition - specified by the equal sign // Both marked with 'extern' My question is - is this a C vs. C++ distinction, or is it a pre-ANSI vs. ANSI practice? Now, the more practical question

问题 I have some macro constants defined in a .c file and a .h file as follows: #define LOCAL_L2_BASE_LV (0x00800000UL) in .c file and #define FPGA_PLI_FEEDBACK_OFFSET_LV_FREQUENCY (0x00007000UL) in .h file I would like to use these constants in a different header file. What is the best way to do so? My initial idea was to use extern in the header file I want to use the constants in, but is this valid? 回答1: Macros can not be made extern, they are not variables. They are simply textual replacements

问题 I am trying to move some functions to separate file in c project. I made util.h file with #ifndef _UTIL_H #define _UTIL_H #include <signal.h> #include <termios.h> #include <time.h> ... extern struct timeval tv1, tv2, dtv; void time_start(); long time_stop(); and I made util.c file with #include "util.h" ... struct timeval tv1, tv2, dtv; void time_start() { gettimeofday(&tv1, &timezone); } long time_stop() { gettimeofday(&tv2, &timezone); dtv.tv_sec = tv2.tv_sec - tv1.tv_sec; ... in cmake I