How can i concatenate a const char with char array? - c++

Question

The array itself is held at
char filestring[9];
and initialized via
snprintf_P(filestring,
sizeof(filestring),
PSTR("%04u%02u%02u"),
dt.Year(),
dt.Month(),
dt.Day());
How can i concatenate all above as in the example? (Add the slash and the .txt extension to the filestring variable)
File file = SD.open("/" + filestring + ".txt", FILE_APPEND);
I get the following misleading error for the example above.
expression must have integral or unscoped enum type

Answer 1

Maybe something like this:
char filename[MAX_PATH] = {0};
int n = snprintf(filename, sizeof(filename), "/%s.txt", filestring);
// check whether snprintf succeeded
if (n > 0 && n < sizeof(filename)) {
File file = SD.open(filename, FILE_APPEND);
}
Update: As requested by a user I am adding a clarification on MAX_PATH:
The line
char filename[MAX_PATH] = {0};
Defines a character array of size MAX_PATH. That could have used any integer value that you thought right for your program but, using MAX_PATH ensures the buffers can hold any filename.
On Linux, you must #include <limits.h> (or you can #include <stdio.h> and use FILENAME_MAX). I am not a Windows user but it looks like you have to #include <stdlib.h> to import MAX_PATH (doc).
Of course you could also also initialized filestring with the desired format in one go:
char filestring[MAX_PATH];
snprintf_P(filestring,
sizeof(filestring),
PSTR("/%04u%02u%02u.txt"),
dt.Year(),
dt.Month(),
dt.Day());

Answer 2

In C:
const int size = MAX_PATH;
char path[size];
int rc = snprintf(path, size, "/%s.txt", filestring);
if (rc < 0) {
fprintf(stderr, "Concatenation error.\n");
} else if (rc > size) {
fprintf(stderr, "Buffer is too small.\n");
} else {
printf("path: %s\n", path);
// Use it...
}
In C++ (since you tagged your question C++):
std::string path = "/" + std::string(filestring) + ".txt";
File file = SD.open(path.c_str(), FILE_APPEND);

Answer 3

Here's an alternative using a std::ostringstream to build the filename and a std::string to pass the result around to other functions:
#include <iomanip>
#include <sstream>
#include <string>
void some_function() {
std::ostringstream os;
// build the string using the std::ostringstream
os << std::setfill('0')
<< '/'
<< std::setw(4) << dt.Year()
<< std::setw(2) << dt.Month()
<< std::setw(2) << dt.Day()
<< ".txt";
// extract the result into a std::string
std::string filestring(os.str());
// Then depending on the SD.open() interface:
// 1. The preferred:
File file = SD.open(filestring, FILE_APPEND);
// 2. Backup version:
File file = SD.open(filestring.c_str(), FILE_APPEND);
}