extern

问题 From Here it seems that default argument are not supported by C. I have the following method in exported library: extern "C" { __declspec (dllexport) uintptr_t Method(int freq, int *pRetval, bool *support2MHz); } If i made last argument optional like this: extern "C" { __declspec (dllexport) uintptr_t Method(int freq, int *pRetval, bool *support2MHz = NULL); } My dll is still compiled. My question is why? Everyone says the default arguments are not supported in C code. I use C++ for MS 2015.

问题 this is hard to explain in text, so I will give an example. //f1.c int a = 5; int main() { printf("func2() output is: %i\n", func2() ); return 0; } //f2.c static int a = 3 int func2() { extern int a; return a; } When I compile and run this, I get 3, while I was expecting 5. Can anyone explain to me why I get 3? I would have thought that by using extern within the function, it wouldn't use the value of the static variable. 回答1: From n1256 §6.2.2 ¶4: For an identifier declared with the storage

问题 this is hard to explain in text, so I will give an example. //f1.c int a = 5; int main() { printf("func2() output is: %i\n", func2() ); return 0; } //f2.c static int a = 3 int func2() { extern int a; return a; } When I compile and run this, I get 3, while I was expecting 5. Can anyone explain to me why I get 3? I would have thought that by using extern within the function, it wouldn't use the value of the static variable. 回答1: From n1256 §6.2.2 ¶4: For an identifier declared with the storage

问题 this is hard to explain in text, so I will give an example. //f1.c int a = 5; int main() { printf("func2() output is: %i\n", func2() ); return 0; } //f2.c static int a = 3 int func2() { extern int a; return a; } When I compile and run this, I get 3, while I was expecting 5. Can anyone explain to me why I get 3? I would have thought that by using extern within the function, it wouldn't use the value of the static variable. 回答1: From n1256 §6.2.2 ¶4: For an identifier declared with the storage

问题 this is hard to explain in text, so I will give an example. //f1.c int a = 5; int main() { printf("func2() output is: %i\n", func2() ); return 0; } //f2.c static int a = 3 int func2() { extern int a; return a; } When I compile and run this, I get 3, while I was expecting 5. Can anyone explain to me why I get 3? I would have thought that by using extern within the function, it wouldn't use the value of the static variable. 回答1: From n1256 §6.2.2 ¶4: For an identifier declared with the storage

问题 I want to create a dynamic library(.dll) for one of the components written in C++, used in my work. Couple of functions are declared as extern in a .h file and being refernced in the corresponding .cpp file. While linking them i'm getting Unresolved symbol error. xxxx.obj : error LNK2019: unresolved external symbol "int __cdecl run_xxx(int,char * * const,struct st_xx *)" (?run_xxx@@YAHHQEAPEADPEAUst_xx@@@Z) referenced in function "int __cdecl start_xx(int,int,int,char * * const)" (?start_xx@

问题 Can anyone please tell me is there any special requirement to use either EXTERN or GLOBAL variables in a C program? I do not see any difference in a program like below, if I change from gloabl to extern. #include <stdio.h> #include <stdlib.h> int myGlobalvar = 10; int main(int argc, char *argv[]) { int myFunc(int); int i; i = 12; myGlobalvar = 100; printf("Value of myGlobalvar is %d , i = %d\n", myGlobalvar, i); i = myFunc(10); printf("Value of passed value : %d\n",i); printf("again Value of

问题 Can anyone please tell me is there any special requirement to use either EXTERN or GLOBAL variables in a C program? I do not see any difference in a program like below, if I change from gloabl to extern. #include <stdio.h> #include <stdlib.h> int myGlobalvar = 10; int main(int argc, char *argv[]) { int myFunc(int); int i; i = 12; myGlobalvar = 100; printf("Value of myGlobalvar is %d , i = %d\n", myGlobalvar, i); i = myFunc(10); printf("Value of passed value : %d\n",i); printf("again Value of

问题 Can anyone please tell me is there any special requirement to use either EXTERN or GLOBAL variables in a C program? I do not see any difference in a program like below, if I change from gloabl to extern. #include <stdio.h> #include <stdlib.h> int myGlobalvar = 10; int main(int argc, char *argv[]) { int myFunc(int); int i; i = 12; myGlobalvar = 100; printf("Value of myGlobalvar is %d , i = %d\n", myGlobalvar, i); i = myFunc(10); printf("Value of passed value : %d\n",i); printf("again Value of

来源： https://stackoverflow.com/questions/52261768/extern-c-with-class-and-dll