converting a variable name to a string in C++

Question

I'd like to output some data to a file. For example assume I have two vectors of doubles:

vector<double> data1(10);
vector<double> data2(10);

is there an easy way to output this to a file so that the first row contains the headings 'data1' and 'data2' followed by the actual contents. The function which outputs the data will be passed various different arrays so hardcoding the name of the heading is not possible - ideally I'd like to convert the variable name to some string and then output that string followed by the contents of the vector array. However, I'm not sure how to convert the variable name 'data1' to a string, or indeed if it can easily be done (from reading the forums my guess is it can't) If this is not possible an alternative might be to use an associative container such as map or perhaps more simply a 'pair' container.

pair<vector<double>,string> data1(10,'data1');

Any suggestions would be welcome!

I have to wonder what you are trying to achieve. If it is arbitrary amounts of data, naming each part of it won't do you any good, because it has to be done by hand anyway. Your second solution is probably the one you are looking for. — data
– data, Commented Aug 2, 2010 at 10:36
Wow, this is a bit trickey, one thing you could try to do is make classes or structs, which have an element that is the string 'name'. — user1478209
– user1478209, Commented Jul 4, 2012 at 16:05

sarnold · Accepted Answer · 2010-08-02 10:40:03Z

85

You can use the preprocessor "stringify" # to do what you want:

#include <stdio.h>

#define PRINTER(name) printer(#name, (name))

void printer(char *name, int value) {
    printf("name: %s\tvalue: %d\n", name, value);
}

int main (int argc, char* argv[]) {
    int foo = 0;
    int bar = 1;

    PRINTER(foo);
    PRINTER(bar);

    return 0;
}


name: foo   value: 0
name: bar   value: 1

(Sorry for printf, I never got the hang of <iostream>. But this should be enough.)

answered Aug 2, 2010 at 10:40

sarnold

105k23 gold badges187 silver badges243 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Maimas2 Over a year ago

Nice. printf is fine in my opinion, just another option for ouput. They're all the same to me.

sarnold Over a year ago

@cs1349459 in a small toy like this, yes, C stdio isn't much different from the C++ iostreams. However, in larger projects, quite often the C++ iostream synchronizing with C stdio takes enough time that it is disabled. Mixing stdio with iostreams in that case will probably cause surprising behavior. I still don't understand iostream well :) so head to en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio for more information.

SaeidMo7 · Accepted Answer · 2016-09-29 16:41:24Z

27

try this:

#define GET_VARIABLE_NAME(Variable) (#Variable)

//in functions

int var=0;    
char* var_name= GET_VARIABLE_NAME(var);

answered Sep 29, 2016 at 16:41

SaeidMo7

1,32418 silver badges23 bronze badges

Comments

Timmy_A · Accepted Answer · 2016-09-06 09:49:02Z

10

I had the same problem. After a little bit of experimentation I created following macros that convert names of variables, fields, functions, methods and types to strings.

#define MACRO_VARIABLE_TO_STRING(Variable) (void(Variable),#Variable)

#define MACRO_FUNCTION_TO_STRING(Function) (void(&Function),#Function)

#define MACRO_METHOD_TO_STRING(ClassName,Method) (void(&ClassName::Method),#Method)

#define MACRO_TYPE_TO_STRING(Type) (void(sizeof(Type)),#Type)

The code uses comma operator and void conversion to force compiler to check if variable, function, etc. really exists. The nice thing is that it works well with uninitialized variables too. I tested it on both VC and GCC with all pedantic options I found out without any warning messages.

int GetAndPrintValue(const char* VariableName)
{
   std::cout << VariableName << std::endl;
   return 10;
}

int Variable=GetAndPrintValue(MACRO_VARIABLE_TO_STRING(Variable));

I use such code when I write parsers that reads data from input stream and if parsed variable is out of bounds it throws an exception with name of variable that failed my validity checks.

answered Sep 6, 2016 at 9:49

Timmy_A

1,24213 silver badges10 bronze badges

1 Comment

uuu777 Over a year ago

An excellent solution in my view

thiagowfx · Accepted Answer · 2024-07-17 09:49:19Z

5

Slightly adapted from @sarnold's answer, for C++:

#define DEBUG(x) std::cerr << #x << " = " << x << std::endl;

An example program which uses this:

int main() {
    int foo = 1;
    DEBUG(foo);

    return 0;
}

edited Jul 17, 2024 at 9:49

answered Apr 6, 2015 at 16:18

thiagowfx

6,0177 gold badges44 silver badges64 bronze badges

2 Comments

Ow Ji Over a year ago

Hey, I use this in my competitive programming template. For a debugging aid it might be better to use std::cerr to distinguish it from stdout.

thiagowfx Over a year ago

Agreed. Updated.

Puppy · Accepted Answer · 2010-08-02 10:30:27Z

3

You can use the preprocessor, there's a stringify token, but it's only available from the source, not to a function (you'd get the argument name).

answered Aug 2, 2010 at 10:30

Puppy

147k40 gold badges271 silver badges480 bronze badges

Comments

Ben · Accepted Answer · 2020-11-23 14:29:22Z

3

I had a similar quest. In Qt, I got tired of constantly writing the variable name as a string without autocomplete when writing to qDebug(). After a lot of trial and error with different macros and functions, I found that this macro works great:

#define PRINT(x) ", " << #x << ": " << x

Example usage:

int someVariable = 42;
double anotherVariable = 13.37;
qDebug().nospace() << "Some text" << PRINT(someVariable) << PRINT(anotherVariable);

Output:

Some text, someVariable: 42, anotherVariable: 13.37

I guess this (or something very similar) will work for std::cout as well.

A bit late to the party, but I hope this can help anyone out there!

answered Nov 23, 2020 at 14:29

Ben

513 bronze badges

Comments

anon · Accepted Answer · 2010-08-02 10:34:15Z

1

I'd have thought the obvious answer is to make the function that performs the output take the heading text as a string parameter.

answered Aug 2, 2010 at 10:34

anon

1 Comment

Wawel100 Over a year ago

Ideally I'd like to be able to call the function that does the output without a string parameter containing the names of the varialbes.

Dmitry Bravikov · Accepted Answer · 2018-02-05 10:35:30Z

1

For this case I have made nameof() macro. It returns a std::string name of a variable, type or member. It works like nameof() in C#.

For Example:

#include "nameof.h"

std::vector<double> data1(10);
std::string name = nameof(data1); // "data1"

struct Foo1
{
    struct Foo2
    {
        Foo1* foo1;
    };

    Foo1* foo1;
    Foo2 foo2;
};

name = nameof(Foo1::foo1->foo2.foo1); // "foo1"

name = nameof(123); // std::logic_error exception

answered Feb 5, 2018 at 10:35

Dmitry Bravikov

1397 bronze badges

Collectives™ on Stack Overflow

converting a variable name to a string in C++

8 Answers 8

2 Comments

Comments

1 Comment

2 Comments

Comments

Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

2 Comments

Comments

1 Comment

2 Comments

Comments

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related