Ability to use Standard Library instead of opentelemetry nostd classes

[maxgolov]

Due to ABI stability requirements OpenTelemetry C++ SDK provides the standard implementation of 'common' std-alike classes under nostd namespace:

• function ref
• shared_ptr
• span
• string_view
• unique_ptr
• variant

However, in some environments it may be of benefit to alias the nostd types to standard library, esp. if compiler has full support of C++17. And if the environment the SDK is being built for has no need to consume any external prebuilt exporters.

One way to achieve this is a custom compile-time build option (cmake flag) that aliases nostd with std namespace. Tests could be added to verify that the SDK still works well with recent version of Visual Studio, Apple llvm-clang, gcc, etc. (in essence, replacing the nostd implementations with std).

[g-easy]

Due to ABI stability requirements OpenTelemetry C++ API provides nostd::*.

What would be required so that e.g. std::string_view could be transparently passed to API functions expecting a nostd::string_view?

[rnburn]

@g-easy - that should be easy. Just add an implicit conversion constructor if the c++ standard is 17 or greater.

[rnburn]

@maxgolov

One way to achieve this is a custom compile-time build option (cmake flag) that aliases nostd with std namespace.

I'd recommend a more granular approach, maybe using directives within the nostd namespace, e.g.

namespace nostd {
using std::string_view;
}

The nostd components were added in different versions of c++ so we'd want to alias only if there're available. For example std::span was added in C++20 and function_ref is a proposal that hasn't been added yet.

[maxgolov]

Thanks Ryan for your feedback. I'll take a look at that option. In most cases I'd expect vs2019 and latest clang to have most of that. function_ref can be kept as exceptional case until (if and when) becomes a standard.

[g-easy]

IIUC Abseil went the using std::string_view; route and this caused ABI issues. I suspect we need a shim that translates std::string_view to nostd::string_view e.g. potentially swapping the pointer and the length fields.

[maxgolov]

@g-easy - I'll share some thoughts on this. I made it work with STL and Visual Studio 2017+ in my fork. ABI compat won't be an issue for customers statically linking both API and SDK in one. One hiccup I have is with nostd::span , which is non-standard yet. gsl::span implementation works equally well though:
https://github.com/isocpp/CppCoreGuidelines/blob/master/docs/gsl-intro.md

[maxgolov]

I'll send a proposed PR with changes later this week. I only tested this on Visual C++ 2019, working on tests for other platforms.

[0x4b]

@maxgolov has there been any progress on allowing STL? The interns contributing context API functionality are finding the nostd requirement to be more of a burden than we expected.

Having clarity around whether STL is banned just at the interface or also in the implementation would be a big help. What do you think about making note in CONTRIBUTING.md (I can send a PR)?

[reyang]

Related to #134.