Using macro definition vs const variable [duplicate]

Question

I have few questions about pre-processor macros. Here is an example of 2 different code pieces to talk about;

tmp.h
#define DEFAULT_STRING "default_value"

tmp.cpp
void tmpFunc(std::string newValue){
    m_stringValue = newValue;
    if(newValue.isEmpty())
        newValue = DEFAULT_STRING; 
}

And the second version

tmp.h
const std::string m_defaultValue = "default_value"

tmp.cpp
void tmpFunc(std::string newValue){
    m_stringValue = newValue;
    if(newValue.isEmpty())
        newValue = m_defaultValue; 
}

So my questions are;

Does the first version increase the binary size (comparing the first second)?
Does the second version consume more memory (comparing the first one)?
What are the performance impacts of both implementations? Which should run faster and why?
Which is preferable/beneficial in which condition?
Is there any better way for implementation of setting the default value?

Thanks.

There are already so many posts about this topic ... one of them is stackoverflow.com/questions/1637332/static-const-vs-define — YesThatIsMyName
– YesThatIsMyName, Commented Apr 16, 2018 at 9:41
No, it's not an exact duplicate. The other question does not ask these specific questions about the two approaches. — DodgyCodeException
– DodgyCodeException, Commented Apr 16, 2018 at 9:57

anatolyg · Accepted Answer · 2018-04-16 09:54:41Z

3

Does the first version increase the binary size (comparing the first second)?

No. Since the string is used only once, it will appear only once in the compiled file.

Does the second version consume more memory (comparing the first one)?

No. It works with memory in pretty much the same manner.

What are the performance impacts of both implementations? Which should run faster and why?

Both implementations copy the string around: the method receives newValue by value, copying it. This copying will dominate the performance, I guess.

However, as always with performance, just measure it - it should be easy, since you already have both versions of the code implemented.

Which is preferable/beneficial in which condition?

Macros are not an idiomatic solution in C++. Use the const string.

Is there any better way for implementation of setting the default value?

Receive the new string by reference. Other than that, not so much. I assume here the default is a short string, like "none" or "default". These strings have short string optimization, so it doesn't matter much how you store it.

As always with performance, if you see that your method is a performance bottleneck, optimize it. If you don't know, don't optimize it.

answered Apr 16, 2018 at 9:54

anatolyg

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1 Comment

benjamin button Over a year ago

I use references very often but is there any significant difference to pass the primitive types by reference?

Bathsheba · Accepted Answer · 2018-04-16 09:45:01Z

2

Both are horrible, the second one marginally better than the first.

The best way is to use

constexpr char DEFAULT_STRING[] = "default_value";

For more on constexpr, see When should you use constexpr capability in C++11?

edited Apr 16, 2018 at 9:45

answered Apr 16, 2018 at 9:38

Bathsheba

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5 Comments

Radim Bača Over a year ago

could you please elaborate more on why it is better?

YSC Over a year ago

Why not a constexpr std::string then?

Bathsheba Over a year ago

@YSC, your compiler will puke on that. string_view was added in C++17 to help.

Öö Tiib Over a year ago

@YSC because std::basic_string does not have constexpr constructors. In idea the constexpr std::string_view foo("abc"); should work but ... does not on majority of compilers yet. So char array is fine at the moment.

YSC Over a year ago

True. Maybe a std::string_view then. At that point, I'm starting to agree with you with the use of char[].

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