error LNK2001: unresolved external symbol

[Igor Vartanov]

I was declared and define in the same file(as per C programing concept). In C++, declartion goes to header file and definitions should be in CPP file.

Actually, C and C++ share the same concept. As per design, C++ was made a successor to C.

Header files contain declarations with external linkage (functions are automatically given the one), while compile units (.c or .cpp) contain definitions. Without external declaration a variable is limited to unit scope, compiler generates allocation code for the variable, and its references appear resolved immediately in the unit scope. External declaration instructs to resolve references in the other units (link them to variable real location) on a later stage by linker. That's why linker is called linker.