C or C++ string formatting library for embedded(!) development

Question

Using C++ std:: we can format strings like this:

 cout << "The x=" << x << endl;

In C# and Java we can build strings like:

 var sb = new StringBuilder(1000);
 sb.Append("The x=");
 sb.AppendLine(x);

For C (or C++ without std::) in embedded world (especially smaller 8 bit controllers) we have: Xprintf family functions or string.h functions. My concerns about them:

1. Xprintf are bulky functions if one count each cycle (that happens often). Even without floating point
2. String.h functions are tedious to use

What I would like to find is some library that can do something like:

char buffer[100];
StringBuilder sb(buffer, 100);
sb.append("The x=");
sb.append(x);

Or in C notation:

char buffer[100];
sb_struct_t sb = string_builder_init(buffer, 100);
sb_append_pchar(sb, "The x=");
sb_append_uint16(sb, x);

So it is pretty thin and simple wrapper around standard lib functions. It looks like a wheel, that I would reinvent if I write it myself.

At the same time I have found many string formatting and streaming libraries, but non that could be use for embedded usages (most of them have a lot of internal memory allocations for example)

So the question:
Do you know any ready to use C/C++ libraries that could fit for embedded development:

1. No dynamic (possible to switch off) memory allocations
2. Small overhead
3. Not using C++ std:: (if C++)
4. Better if especially designed for embedded usage

Answer 1

• Have you considered using the fmt library with the BasicArrayWriter?
• How about the FastFormat library using a char Buffer Sink?

That would seem to suit your needs of building/composing/appending to a string within a fixed buffer. fmt is dependent on the standard library, so you could only use it if you replace some of those dependencies (vectors, sets etc.) - although it's probably the case that the part of the code you're interested in actually depends on a lot less. Maybe you could carve it out. FastFormat seems to have a lot less standard library dependencies, and a lot of the external dependencies it does have seem to restricted to specific sinks, so it might even possible to even build it with no standard library dependence, although I would guess it would need some elbow grease as well.

Answer 2

If you need to use floating point or a number of formats/bases then your best bet is the snprint function which is "safer" for embedded use as you pre-declare the buffer to use and the function is size limited to avoid overflows. As well as flexibility it has the advantage of portability since it is a part of the ANSI standard for C/C++.

If you only have a very limited number of formats, (not including floats), then the itoa function supported by some/most compilers is usually your best bet but it is not always available as it is not in the ANSI standard.