Does non- extern const have internal linkage?
in [C++]
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From: Eric Pruneau on 8 Mar 2010 05:04 "matt" <lists(a)givemefish.com> a �crit dans le message de news: 14f75a01-536a-41bd-9f28-699f87087098(a)t23g2000yqt.googlegroups.com... > Hi all, > > I was reading in Bruce Eckel's Thinking in C++, 2nd Ed Vol1, Ch 8 > about extern and const. He states that: > > "To use const instead of #define, you must be able to place const > definitions inside header files as you can with #define. This way, you > can place the definition for a const in a single place and distribute > it to translation units by including the header file. A const in C++ > defaults to internal linkage; that is, it is visible only within the > file where it is defined and cannot be seen at link time by other > translation units." > > However, I wrote a short test program in gcc 4.4.1 that defines a non- > extern constant within a header file as: > > const double myConst = 3.14159; > > This header is included by another file, which then uses the value of > myConst without problem. The fact that this is allowed seems to > contradict the assertions made by Eckel. The important thing here is translation unit... A translation unit is the basic unit of compilation in C++. It consists of the contents of a single source file, plus the contents of any header files directly or indirectly included by it, minus those lines that were ignored using conditional preprocessing statements. > So, is gcc allowing this although it shouldn't? Or is Mr. Eckel's > statement incorrect? Or have I misunderstood something? > Example: -----file1.cpp---- #include "file1.h" ...... double twoPi = 2*myConst; // ok defined in file2.h which is in the same translation unit -----file1.h----- #include "file2.h" ..... -----file2.h------- const double myConst = 3.14159; ..... Eric Pruneau -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Daniel Krügler on 8 Mar 2010 12:20
On 8 Mrz., 23:06, Daniel Kr�gler <daniel.krueg...(a)googlemail.com> wrote: > On 8 Mrz., 20:15, matt <li...(a)givemefish.com> wrote: > External variables (including constants with external > linkage) can only be defined once per program without > violating the one-definition rule. ... unless they are provided inside an unnamed-namespace: namespace { int var; } Greetings from Bremen, Daniel Kr�gler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |