C++ requires that the size of an array be a constant integral expression. However, the following code refuses to compile:

 // in const.hextern const int ID_LENGTH; // declaration// in const.cppconst int ID_LENGTH=1024; // definition// in main.cpp#include int main(){ char id[ID_LENGTH];  // compilation error }

There are two problems with this code. First, the value of the constant ID_LENGTH isn’t known when the compiler sees the definition of the array in main.cpp. Because of this, the compiler cannot compute the array’s size. Secondly, the declaration and definition of ID_LENGTH are incompatible: while the declaration says that ID_LENGTH has external linkage, in the definition, ID_LENGTH is implicitly defined as having static (i.e., file-scope) linkage. To fix this, you can remove the extern specifier from the declaration and change it to a definition by initializing the constant:

 // in const.hconst int ID_LENGTH=1024; // declaration becomes definition                          // linkage type changed as well