Problem with c++

ESPHome
1

Hello,
I am currently trying to outsource some functions to an external C++ file. I have come quite far with it, but unfortunately I am not familiar enough with C++. The file I have created looks like this:

#include <cstdio>
#include "esp_log.h"

static const char* TAG = "converters.ccp";

const char * float_to_char(float input) {
    char buffer[64];
    int ret = snprintf(buffer, sizeof buffer, "%f", input);

    if (ret < 0) {
        // error while converting
        ESP_LOGE(TAG, "Error while converting float to char [%.1f%%]");
        return "";
    }
    if (ret >= sizeof buffer) {
        // Result was truncated - resize the buffer and retry.
        ESP_LOGW(TAG, "Result was truncated while converting float to char [%.1f%%]");
    }

    return buffer;
}

I get the following error message:

src/converters.cpp: In function 'const char* float_to_char(float)':
src/converters.cpp:7:10: warning: address of local variable 'buffer' returned [-Wreturn-local-addr]
     char buffer[64];
          ^~~~~~

When I change the definition of buffer to “char *” I get the following error message:

src/converters.cpp: In function 'const char* float_to_char(float)':
src/converters.cpp:8:24: error: cannot convert 'char**' to 'char*'
     int ret = snprintf(buffer, sizeof buffer, "%f", input);
                        ^~~~~~
In file included from /data/cache/platformio/packages/toolchain-xtensa-esp32/xtensa-esp32-elf/include/c++/8.4.0/cstdio:42,
                 from src/converters.cpp:1:
/data/cache/platformio/packages/toolchain-xtensa-esp32/xtensa-esp32-elf/sys-include/stdio.h:266:15: note:   initializing argument 1 of 'int snprintf(char*, size_t, const char*, ...)'
 int snprintf (char *__restrict, size_t, const char *__restrict, ...)
               ^~~~~~
src/converters.cpp:20:12: error: cannot convert 'char**' to 'const char*' in return
     return buffer;
            ^~~~~~

What am I doing wrong?

2

You define a variable inside your function. So the scope is only inside the function. Once the function is exited, the memory space for the variable is returned to the free space. Anything that comes after the function can be assigned that memory again.

So make the buffer global or make it static. Or better, declare the buffer where you would like to call your function and pass it to the function. Basically how snprintf() already works but now you include ESPHome logging.

3

Just curious. This is just to learn something ?
Or You really write code already exists.
Why not to use to_string() ?