C++: undefined reference to const pointer when declaring as extern

Question

I get a compiler error when programming the following:

file1.cpp: declaring/defining of several variables
file1.cpp: defining a pointer array that is pointing to each of these variables -> this array shall be const (=always point to these variables)
file2.cpp: here I want to use the pointer array and use the variables it points to.

    //file1.cpp
    int a,b,c,d;
    int *const pa[4] = {&a, &b, &c, &d};

    //file2.cpp
    extern int *const pa[4];

when compiling it drops the error in file2.cpp:

undefined reference to `pa'

How to define that pointer array with constant pointers and use it in different source files?

Best regards :-)

VLL · Accepted Answer · 2020-02-26 07:25:24Z

1

A const variable has internal linkage by default. To get external linkage, add extern to the definition:

extern int *const pa[4] = {&a, &b, &c, &d};

answered Feb 26, 2020 at 7:25

VLL

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2 Comments

walnut Over a year ago

Or probably cleaner: Put the extern int *const pa[4]; declaration in a header and include it in both file1.cpp and file2.cpp. This avoids type mismatch.

mostrich Over a year ago

Thank you. It's working now. Knowing this, after another 5min research I found, that what I did above is working with variables. Using a const always requires an extern even at the definition. Or as walnut mentioned to put it in a header.

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