use a const variable as opposed to #define - Multiple const objects created
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Recently, I try to replace #define with const in header file. However, there are concerns on, multiple const object will be created, if the header file is included in multiple cpp files. For example: In version.h ------------ #ifndef VERSION #define VERSION #include const std::string version("alpha_0-22"); #endif In main.cpp ------------ #include #include "version.h" extern void fun(); int main() { printf("address of version in main=%p\n", &version); fun(); getchar(); } In fun.cpp ------------ #include #include "version.h" void fun() { printf("address of version in fun=%p\n", &version); } The output of the program will be: address of version in main=00431960 address of version in fun=00431984 It seems that two copies of version string had been created if version.h is included in different cpp file scope. Now I am worry if version.h file is included in thousand of cpp files, will thousand of version string object be created?! My alternative workaround on this is, I will let version.h declare the version string and version.cpp define the version string. In version.h ------------ #ifndef VERSION #define VERSION #include extern const std::string version; #endif In version.cpp -------------- #include "version.h" const std::string version("alpha_0-22"); Again, here is my output: address of version in main=00431960 address of version in fun=00431960 It seems that the const string just be constructed one time only. I am not sure whether this is the correct workaround? Or my concern on multiple creation of const object is not an issues? Please refer to http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.7 on why I am using const instead of const. Thank you for your feedback. yccheok
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Recently, I try to replace #define with const in header file. However, there are concerns on, multiple const object will be created, if the header file is included in multiple cpp files. For example: In version.h ------------ #ifndef VERSION #define VERSION #include const std::string version("alpha_0-22"); #endif In main.cpp ------------ #include #include "version.h" extern void fun(); int main() { printf("address of version in main=%p\n", &version); fun(); getchar(); } In fun.cpp ------------ #include #include "version.h" void fun() { printf("address of version in fun=%p\n", &version); } The output of the program will be: address of version in main=00431960 address of version in fun=00431984 It seems that two copies of version string had been created if version.h is included in different cpp file scope. Now I am worry if version.h file is included in thousand of cpp files, will thousand of version string object be created?! My alternative workaround on this is, I will let version.h declare the version string and version.cpp define the version string. In version.h ------------ #ifndef VERSION #define VERSION #include extern const std::string version; #endif In version.cpp -------------- #include "version.h" const std::string version("alpha_0-22"); Again, here is my output: address of version in main=00431960 address of version in fun=00431960 It seems that the const string just be constructed one time only. I am not sure whether this is the correct workaround? Or my concern on multiple creation of const object is not an issues? Please refer to http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.7 on why I am using const instead of const. Thank you for your feedback. yccheok
Place: #pragma once at the very beginning of the header file. This instructs the compiler to only compile the header file once.
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Place: #pragma once at the very beginning of the header file. This instructs the compiler to only compile the header file once.
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Recently, I try to replace #define with const in header file. However, there are concerns on, multiple const object will be created, if the header file is included in multiple cpp files. For example: In version.h ------------ #ifndef VERSION #define VERSION #include const std::string version("alpha_0-22"); #endif In main.cpp ------------ #include #include "version.h" extern void fun(); int main() { printf("address of version in main=%p\n", &version); fun(); getchar(); } In fun.cpp ------------ #include #include "version.h" void fun() { printf("address of version in fun=%p\n", &version); } The output of the program will be: address of version in main=00431960 address of version in fun=00431984 It seems that two copies of version string had been created if version.h is included in different cpp file scope. Now I am worry if version.h file is included in thousand of cpp files, will thousand of version string object be created?! My alternative workaround on this is, I will let version.h declare the version string and version.cpp define the version string. In version.h ------------ #ifndef VERSION #define VERSION #include extern const std::string version; #endif In version.cpp -------------- #include "version.h" const std::string version("alpha_0-22"); Again, here is my output: address of version in main=00431960 address of version in fun=00431960 It seems that the const string just be constructed one time only. I am not sure whether this is the correct workaround? Or my concern on multiple creation of const object is not an issues? Please refer to http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.7 on why I am using const instead of const. Thank you for your feedback. yccheok
#ifndef, #define and #endifcombination works to prevent redundant inclusion in the same file. This will not work if you are including them in different.cppfiles. Since in that.cppfile the macro that you are checking is not defined. Hence it creates anotherstd::string versionvar. What you have to do is create a global variable by declaring it in a header file which will be included everywhere (for examplestdafx.hin VC6) like wiseextern std::string version;
and then defining it in
any one and only one of the **.cpp**files. Now you can use the variable by including the header file where you wish to use the variable. Do read the faqs[^] too.
Nibu thomas Software Developer