C++ float to string conversion failing with stringstream

Question

I am working on the particle.io Spark platform, currently stuck trying to print out a float as a string. I saw some solutions where you use a string stream to convert the float.

My implementation is as follows below:

#include <sstream>

void loop()
{
    float tempC = 21.35;

    std::ostringstream stream;
    stream << tempC;
    std::string tempCString = stream.str();

    // why does this give me a blank string?
    Serial.print("Temp 1: ");
    Serial.println(tempCString.c_str());

    // while this outputs the float
    Serial.print("Temp 2: ");
    Serial.println(tempC);

    Serial.println(tempCString.size());
}

This results in the following output:

Temp 1:
Temp 2: 21.35
6

Also, this fails to compile:

Serial.println(tempCString);

With the following error:

error: no matching function for call to 'USBSerial::println(std::string&)'

Edit: link to particle docs about Serial.print

https://docs.particle.io/reference/firmware/core/#print-

May be you should provide a [MCVE] that reproduces your problem and leave out the non standard parts. — πάντα ῥεῖ, Oct 13 '16 at 07:58
clarified the code to make it more complete, minimal, verifiable — koverda, Oct 13 '16 at 08:03
Well, then, Treyco .... how about you explain what `Serial` does. It is non-standard, and from (lack of) information given could well be the culprit. — Peter, Oct 13 '16 at 08:04
@Peter Its an arduino function used to print like printf..., if you don't know about the API he's using you obviously won't find the answer — Treycos, Oct 13 '16 at 08:06
print and println output strings without problems, so i would guess the issue is with converting the float to a string. — koverda, Oct 13 '16 at 08:09
`c_str()` should work since `"Temp 1: "` works whatever the API. I replaced `println` by a mere `printf` and first part worked fine... — Jean-François Fabre, Oct 13 '16 at 08:09
@koverda: have you tried the `sprintf(temp_string,"%f",tempC)` ? — Jean-François Fabre, Oct 13 '16 at 08:10
koverda: Did you mean `Serial.println(tempCString);` fails to compile? (It would match your error message better). If so, that just means `println` has no overloads which take a `std::string` - you need to call `.c_str()` first. — Martin Bonner supports Monica, Oct 13 '16 at 08:11
@MartinBonner not exactly sure what you mean by calling .c_str() first. — koverda, Oct 13 '16 at 08:14
"first" was probably unhelpful. You have to do what you did: instead of `Serial.println(str);` use `Serial.println(str.c_str());` — Martin Bonner supports Monica, Oct 13 '16 at 08:15
did you mean that `Serial.println(tempCString.c_str());` will compile? Indeed it does, but it does not print the float as a string for me. — koverda, Oct 13 '16 at 08:15
Also - adding something else to the stringstream doesn't change the output. `stream << tempC << "hello";` still leads to an output of `Temp 1:` — koverda, Oct 13 '16 at 08:17
@koverda: Try `Serial.println(tempCString.size());` - if that outputs "0", there is something strange going on. Can you get a debugger on this thing? If not, add a `std::flush`?? (that is clutching at straws). — Martin Bonner supports Monica, Oct 13 '16 at 08:17
however `stream << "hi" << tempC << "hello";` leads to an output of `Temp 1: hi` — koverda, Oct 13 '16 at 08:19
@MartinBonner `Serial.println(tempCString.size());` gives an output of 6, which makes sense - 5 chars and the terminator — koverda, Oct 13 '16 at 08:22
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/125568/discussion-between-martin-bonner-and-koverda). — Martin Bonner supports Monica, Oct 13 '16 at 08:22
@MartinBonner not sure if i can really get a debugger going, as this is running on a little piece of hardware sitting on my desk. — koverda, Oct 13 '16 at 08:24

score 0 · Accepted Answer · answered Oct 16 '16 at 05:22

I found the answer. Not sure if this holds true for regular c++, but for the version of c++ running on the particle spark, the solution is as follows:

float myFloat = 6.123;
String floatString(myFloat, 2);

This gives me a string that I can work with!

C++ float to string conversion failing with stringstream

1 Answers1