How do I convert a number into a string ?

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This is probably a stupid question but i have not managed to find a working solution to convert an integer to a string so as to display the value on a screen

What I have tried:

I put this code in my source to first make a test with a static value but i plan later to make it a variable.

uint32_t i = 88;//Manual Test
auto charge = std::to_string(i);//Conversion from Figure to String
SimpleTextDraw(lcd_get_width() / 2, lcd_get_height() / 4 + 35, charge, WHITE_TEXT, 100, LAYER_FRONT);

this gives me 2 errors

no suitable conversion function from 'std::string' to 'constant char' exists
and
cannot convert 'std::string' to 'constant char'

Posted 30-Jul-24 2:33am

Carbonkevlar13

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4 solutions

Solution 1

Quote:
no suitable conversion function from 'std::string' to 'constant char' exists
and
cannot convert 'std::string' to 'constant char'

use the c_str method, i.e.

C++

SimpleTextDraw(lcd_get_width() / 2, lcd_get_height() / 4 + 35, charge.c_str(), WHITE_TEXT, 100, LAYER_FRONT);

See, for instance std::basic_string<CharT,Traits,Allocator>::c_str - cppreference.com[^]

Posted 30-Jul-24 2:39am

CPallini

Comments

Carbonkevlar13 30-Jul-24 10:47am

Thanks but the c_str method is not very clear for me

Solution 2

You have the following lines:

C++

auto charge = std::to_string(i);//Conversion from Figure to String
SimpleTextDraw(lcd_get_width() / 2, lcd_get_height() / 4 + 35, charge, WHITE_TEXT, 100, LAYER_FRONT);

In the first line the call to std::to_string returns (and sets charge equal to) a new std::string. In the second line the third parameter, according to the error message, is expected to be a constant char. So you cannot pass a string when it expects a character.

Posted 30-Jul-24 2:43am

Richard MacCutchan

Comments

Carbonkevlar13 30-Jul-24 10:46am

Thanks Richard although that does not help me much..

Richard MacCutchan 30-Jul-24 11:04am

What do you mean, that does not help me much.? Do you not understand the difference between a std::string and a char type, or is it somethng else. Also we are working in the dark somewhat as all we have is two lines of code out of context. So please provide the actual definition of the SimpleTextDraw function.

merano99 30-Jul-24 12:37pm

He probably doesn't know how to convert the result into a const char*.

Richard MacCutchan 30-Jul-24 12:39pm

Or into a "constant char" (without the asterisk). But as so often we only get half the story and have to make guesses.

merano99 30-Jul-24 13:12pm

Since it converts the number 88 into a string and passes the result to a function, I assumed more than one character. I think that a char* is needed. The prototype of the function would help here, but unfortunately, as so often, important details are missing.

Richard MacCutchan 30-Jul-24 12:41pm

Also this yet another example of why auto should be in documentation that no one with less than ten years experience is allowed to read. :)).

merano99 30-Jul-24 13:17pm

Unfortunately, beginners in particular use auto because they assume that the compiler will use the right one. However, the auto data type is not necessarily the problem here. I actually wanted to suggest staying with C at first, as that might be easier for beginners.

Richard MacCutchan 30-Jul-24 14:50pm

I think the use of auto is fine if you know what you are doing. But for beginners it just means they don't always know what the real type is. This is a case in point, he uses auto and assumes he gets "88" returned, when in fact he gets a std::string, but doesn't understand what to do next.

Carbonkevlar13 1-Aug-24 4:33am

Hello All. Thanks a lot for your help and comments. Not only i am a beginner but i do not start from scratch but from an existing program developped to display informations on a screen. So i have to start from the existing program, try to understand what it does and try to make it display what i want. I had found out the auto declaration on some forum trying to go forward. The conversion now works and it will help me a lot as i will receive numbers from a CAN drive to display them on the screen
Thanks again

Solution 4

To manage the memory automatically and prevent memory leaks, you could use a std::unique_ptr (instead of new and delete).

C++

// Conversion to a mutable char* 
auto charge = std::to_string(i); // Conversion from number to string
auto str = std::make_unique<char[]>(charge.size() + 1);
std::copy(charge.begin(), charge.end(), str.get());

SimpleTextDraw(lcd_get_width() / 2, lcd_get_height() / 4 + 35, str.get(), WHITE_TEXT, 100, LAYER_FRONT);

Posted 1-Aug-24 14:59pm

merano99

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merano99 · Accepted Answer · 2024-07-30T04:40:00

Solution 3

To access the content of a std::string, you normally use the c_str() method. This returns a const char*, and the immutable text cannot be used as a mutable char* without explicit type conversion. Since explicit type conversion is not recommended, the best option is to copy the text into a mutable char*.

C++

auto charge = std::to_string(i); // Conversion from number to string
char* str = new char[charge.size() + 1];
strcpy_s(str, charge.size() + 1, charge.c_str());
SimpleTextDraw(lcd_get_width() / 2, lcd_get_height() / 4 + 35, str, WHITE_TEXT, 100, LAYER_FRONT);
delete[] str; // Free up memory!

Posted 30-Jul-24 4:40am

merano99

Comments

Carbonkevlar13 30-Jul-24 10:45am

Thanks. This works although as a newbie i do not understand what it does.
I thought that in the first line "charge" would contain the text "88" coming from i=88 above
What are the other lines doing ?

Carbonkevlar13 30-Jul-24 10:48am

Can i now use this inside a loop where i will vary from 0 to 100 so that i could display this as a counter or a coutdown ?

merano99 30-Jul-24 12:33pm

Even as a newcomer, you can't avoid reading the documentation, e.g. on strings. The string object “charge” contains the text “88”, correct, but your function seems to require an old (non-constant) char pointer. It would be much easier if you could change the function to const char*, but this way there is a type conflict that my code tries to work around. Of course you can use the code inside a loop. If you know the maximum length it would also be possible to request the memory only once with new and release it again at the end.

Carbonkevlar13 1-Aug-24 4:27am

Even when i read the documentation it is not always clear. Thanks