Is there a way to write a large number in C++ source code with spaces to make it more readable? [duplicate]

问题

Imagine I have the code:

vector<int> temp = vector<int>(1 000 000 000);

The above will not compile as the compiler will complain about the spaces. Is it possible to indicate to C++ to ommit those spaces when compiling, or otherwise make the number easier to read?

回答1:

Try digit separator:

int i = 1'000'000'000;

This feature is introduced since C++14. It uses single quote (') as digit separator.

Also see:

Why was the space character not chosen for C++14 digit separators?
Generalizing Overloading for C++2000 (April's joke by the father of C++ himself)

回答2:

When I've done similar things on platforms without C++14 (generally for microprocessors), I've represented large numbers by splitting it up with multiplication:

int i = (1000 * 1000 * 1000);

_{^{Add UL or L postfixes to taste}}

The advantage here is that it's compliant to basically any platform that supports C89 (and probably earlier).

Generally, it's probably safe to assume the multiplication operators will fall out at compile time, but if you're using constants like this in a loop, it might be worth double-checking.

回答3:

I usually #define constants for this purpose, as it saves counting zeroes and makes it very clear what you mean to anyone viewing the code. For example

#define THOUSAND 1000
#define MILLION 1000000

vector<int> temp = vector<int>(THOUSAND * MILLION);

This makes it clear I really do mean a thousand million and did not miscount the zeros

Obviously you can use enums if you prefer.

回答4:

If you don't use C++14, another option would be using some kind of string-inherited class with an implicit int-cast and maybe a regex-check in the constructor to restrict the numbers. I use CString for an easy example.

class NumString : public CString
{
public:
    NumString(CString number) : num(number) { } //maybe insert some regex-check here
    operator long() const
    {
        CString tmp = num;
        tmp.Remove(' ');        
        return atol(tmp);
    }
private:
    CString num;
};


NumString a = "1 000 000 000";
int b = a;
bool test = b == 1000000000;
//test will be true

回答5:

Another idea could be:

#define _000 *1000

int k = 1 _000 _000;

回答6:

As this reminds me of digit grouping my first clumsy approach without C++14
would be

#define INTGROUPED1(a)     (a%1000)
#define INTGROUPED2(a,b)   (a%1000*1000 + b%1000)
#define INTGROUPED3(a,b,c) (a%1000*1000000 + b%1000*1000 + c%1000)
int v1 = INTGROUPED1(        123);
int v2 = INTGROUPED2(    123,123);
int v3 = INTGROUPED3( 23,123,123);

but I would use such tricks rather in a private context.

Just consider someone writing

INTGROUPED3(1234,1234,1234); //This would be (234,234,234) without a compiler complaining

EDIT1:

Maybe a better aproach would be using the ## preprocessor operator

#define NUM_GROUPED_4ARGS(a,b,c,d) (##a##b##c##d)
int num = NUM_GROUPED_4ARGS(-2,123,456,789); //int num = (-2123456789);

This is more like WYSIWYG but not immune against misuse. E. g. you might wnat the compiler to complain about

int num = NUM_GROUPED_4ARGS(-2,/123,456,789);  //int num = (-2/123456789);

but it will not.

来源：https://stackoverflow.com/questions/50559636/is-there-a-way-to-write-a-large-number-in-c-source-code-with-spaces-to-make-it

标签

c++

syntax

numbers