Why does const int change values

Question

Im starting to learn c++ and was under the impression that by putting const is means that the value wont change but i wrote the following code:

#include<iostream>

int main()
{
    const int a = 1;
    a += 1;
    std::cout << a << std::endl;
    return 0;
}

and it prints out 2 while i thought it would have given me an error for changing a const int value. I am using MSVS as my compiler

EDIT: I get a compiler warning saying C4530: C++ exception handler used, but unwind semantics are not enabled, specify /EHsc

It works now and gives me the correct error but does anyone know what this means

"I am using MSVS as my compiler" - That's your first problem. — David G
– David G, Commented Feb 15, 2015 at 21:53
That should definitely not compile, on any version of VS I know. Please double check that you're building and executing the right project, and try rebuilding it. Also take a look at the output of the build, since IIRC you can also run a previous build if the current build fails. — dyp
– dyp, Commented Feb 15, 2015 at 21:55
No, MSVC doesn't give that. It gives error C3892: 'a' : you cannot assign to a variable that is const. Online documentation for that error shows that it existed at least in the VS2005 version. — user743382
– user743382, Commented Feb 15, 2015 at 22:00
It's worth noting that in general, const is a plea to your compiler not to let you (nor anyone maintaining the code) assign to the variable. Technically the value represented by this identifier might be altered by subtle memory corruption (design errors in your system) or malfunction (hardware, etc). As written, your example zooms in on a much simpler case that indeed should not be allowed by valid compilers. Is it possible that you are abridging the real complexity of the source you've provided to the compiler? — Brian Cain
– Brian Cain, Commented Feb 15, 2015 at 22:01
This is such a simple program. Any compiler not giving an error for such simple code would be considered so broken, it would be unusable. Having said this, the code gives the appropriate error for VS 2010. — PaulMcKenzie
– PaulMcKenzie, Commented Feb 15, 2015 at 22:03

ezaquarii · Accepted Answer · 2015-02-15 21:56:06Z

3

This program cannot be compiled using GNU GCC 4.8:

alioth% g++ x.cpp 
x.cpp: In function ‘int main()’:
x.cpp:6:7: error: assignment of read-only variable ‘a’
 a += 1;

Either your compiler is broken or you are doing something wrong (like compiling different project).

answered Feb 15, 2015 at 21:56

ezaquarii

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TriHard8 · Accepted Answer · 2015-02-15 22:00:19Z

0

This program cannot be compiled on VS2013:

1>------ Build started: Project: SOTesting, Configuration: Release Win32 ------
1>  Source.cpp
1>Source.cpp(6): error C3892: 'a' : you cannot assign to a variable that is const
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========`

answered Feb 15, 2015 at 22:00

TriHard8

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Cheers and hth. - Alf · Accepted Answer · 2015-02-15 23:17:29Z

0

The posted code:

#include<iostream>

int main()
{
    const int a = 1;
    a += 1;
    std::cout << a << std::endl;
    return 0;
}

The claim that this code compiled and produced "2" as output, is incorrect.

You can easily get the impression of something like that by inadvertently compiling a different program, or not noticing that a compilation failed and then running an existing executable.

answered Feb 15, 2015 at 23:17

Cheers and hth. - Alf

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