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Add string_view API for c++17 #410

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6c3251f
Add string_view API for c++17
roligugus Oct 16, 2020
e135131
Add features documentation
roligugus Nov 24, 2020
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43 changes: 43 additions & 0 deletions book/src/build/cargo.md
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Expand Up @@ -144,6 +144,49 @@ manifest key][links]* in the Cargo reference.

[links]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key

## Features
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Not sure if that's the place where you'd want to document features if at all. Just a placeholder.

Nicer to look at page: https://github.com/roligugus/cxx/blob/string_view/book/src/build/cargo.md#features


CXX provides features to enable additional functionality such as compiling
the CXX lib with a different standard.

Features are enabled on the CXX crate for Cargo-based setups, and with compiler
options for non-Cargo setups.

The following features are available, see `[features]` section in [Cargo.toml](https://github.com/dtolnay/cxx/blob/master/Cargo.toml) for a full list:
<table>
<tr><th>feature</th><th>compiler option</th><th>description</th></tr>
<tr><td>c++14</td><td>-std=c++14 (gcc, clang), /std:c++14 (msvc)</td><td>Compile CXX lib with c++14</td></tr>
<tr><td>c++17</td><td>-std=c++17 (gcc, clang), /std:c++17 (msvc)</td><td>Compile CXX lib with c++17</td></tr>
</table>

## Additional C++ standard features
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Potential documentation for enabling additional C++ APIs.

Nicer to look at page:
https://github.com/roligugus/cxx/blob/string_view/book/src/build/cargo.md#additional-c-standard-features

Some addition APIs are available if compiling your code with a newer C++ standard.

Please note, that these additional APIs are automatically enabled in the `cxx.h`
header if your code is compiled with the required minimal C++ standard. They do
not depend on building the actual CXX lib against the newer standard at this time.

<table>
<tr><th>Minimal C++ standard</th><th>Description</th></tr>
<tr><td>c++17</td><td>Enables additional `string_view` APIs for `rust::str` and `rust::String`</td></tr>
</table>

Example to e.g. enable the `string_view` APIs for your code, build at least with C++17:
```rust,noplayground
// build.rs

fn main() {
cxx_build::bridge("src/main.rs") // returns a cc::Build
.file("src/demo.cc")
.flag_if_supported("-std=c++17")
.compile("cxxbridge-demo");

println!("cargo:rerun-if-changed=src/main.rs");
println!("cargo:rerun-if-changed=src/demo.cc");
println!("cargo:rerun-if-changed=include/demo.h");
}
```

<br><br><br>

# Advanced features
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55 changes: 55 additions & 0 deletions include/cxx.h
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Expand Up @@ -15,6 +15,45 @@
#include <basetsd.h>
#endif

#ifdef CXXBRIDGE_HAS_STRING_VIEW
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Using CXXBRIDGE_.. naming to align more with other defines. WIP used CXX_... naming

Bikeshedding time?

#error "string_view gets auto-detected"
#endif

// clang-format off
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Disabling clang-format as settings will flatten everything which makes this hard to follow.

Using normal feature detection for compilers supporting it. Feature detection was also noted in the WIP PR.
Tested paths on godbolt.org as I don't have access to all the compiler versions.

Idea is that if string_view is supported, we should support it.

// detect if string_view supported (C++17 required)
#ifdef __has_include
#if __has_include(<version>)
// gcc >= 9, clang >= 9, msvc >= 19.22
#include <version>
#if __cpp_lib_string_view >= 201603L
#define CXXBRIDGE_HAS_STRING_VIEW
#endif
#else
// no <version> include
#if defined(__cpp_lib_string_view) && __cpp_lib_string_view >= 201603L
// gcc: 8, clang: 5 - 8, msvc: 19.15 - 19.21
#define CXXBRIDGE_HAS_STRING_VIEW
#else
// only include if compiled with c++17
#if (__GNUC__ >= 7 && __cplusplus >= 201703L) || (_MSVC_LANG >= 201703L)
// gcc: 7, msvc: 19.14
#define CXXBRIDGE_HAS_STRING_VIEW
#endif
#endif
#endif
#else
// no __has_include
#if _MSC_VER >= 1910L && _MSVC_LANG > 201402L
// msvc: 19.10 requires c++latest (!)
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Older msvc compilers that supported string_view. Not sure what compiler versions you want to support, but I've included the ones I could get my hands on through godbolt.

There could be a discussion if it would make sense to support "c++latest" in cxx as a feature (for msvc).

#define CPPBRIDGE_HAS_STRING_VIEW
#endif
#endif
// clang-format on

#ifdef CXXBRIDGE_HAS_STRING_VIEW
#include <string_view>
#endif

namespace rust {
inline namespace cxxbridge1 {

Expand All @@ -34,6 +73,9 @@ class String final {
String(String &&) noexcept;
~String() noexcept;

#ifdef CXXBRIDGE_HAS_STRING_VIEW
String(std::string_view sv) : String(sv.data(), sv.length()) {}
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We should add "always inline", e.g. __attribute__((always_inline)) (gcc/clang) and __forceinline (msvc), to make sure it gets inlined even if e.g. people disable inlining, etc.

#endif
String(const std::string &);
String(const char *);
String(const char *, size_t);
Expand All @@ -42,6 +84,11 @@ class String final {
String &operator=(String &&) noexcept;

explicit operator std::string() const;
#ifdef CXXBRIDGE_HAS_STRING_VIEW
explicit operator std::string_view() const noexcept {
return {this->data(), this->size()};
}
#endif

// Note: no null terminator.
const char *data() const noexcept;
Expand Down Expand Up @@ -71,13 +118,21 @@ class String final {
class Str final {
public:
Str() noexcept;
#ifdef CXXBRIDGE_HAS_STRING_VIEW
Str(std::string_view sv) : Str(sv.data(), sv.length()) {}
#endif
Str(const std::string &);
Str(const char *);
Str(const char *, size_t);

Str &operator=(const Str &) noexcept = default;

explicit operator std::string() const;
#ifdef CXXBRIDGE_HAS_STRING_VIEW
explicit operator std::string_view() const noexcept {
return {this->data(), this->size()};
}
#endif

// Note: no null terminator.
const char *data() const noexcept;
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10 changes: 10 additions & 0 deletions tests/ffi/lib.rs
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Expand Up @@ -216,8 +216,10 @@ pub mod ffi {
fn r_take_ref_r(r: &R);
fn r_take_ref_c(c: &C);
fn r_take_str(s: &str);
fn r_take_empty_str(s: &str);
fn r_take_slice_char(s: &[c_char]);
fn r_take_rust_string(s: String);
fn r_take_empty_rust_string(s: String);
fn r_take_unique_ptr_string(s: UniquePtr<CxxString>);
fn r_take_ref_vector(v: &CxxVector<u8>);
fn r_take_ref_empty_vector(v: &CxxVector<u64>);
Expand Down Expand Up @@ -470,10 +472,18 @@ fn r_take_str(s: &str) {
assert_eq!(s, "2020");
}

fn r_take_empty_str(s: &str) {
assert_eq!(s.len(), 0);
}

fn r_take_rust_string(s: String) {
assert_eq!(s, "2020");
}

fn r_take_empty_rust_string(s: String) {
assert_eq!(s.len(), 0);
}

fn r_take_slice_char(s: &[c_char]) {
assert_eq!(s.len(), 5);
let s = cast::c_char_to_unsigned(s);
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16 changes: 16 additions & 0 deletions tests/ffi/tests.cc
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Expand Up @@ -624,8 +624,24 @@ extern "C" const char *cxx_run_test() noexcept {
r_take_unique_ptr(std::unique_ptr<C>(new C{2020}));
r_take_ref_c(C{2020});
r_take_str(rust::Str("2020"));
r_take_empty_str(rust::Str{""});
r_take_empty_str(rust::Str{nullptr, 0});
#ifdef CXXBRIDGE_HAS_STRING_VIEW
r_take_str(std::string_view{"2020"});
r_take_empty_str(std::string_view{});
r_take_empty_str(std::string_view{""});
ASSERT(static_cast<std::string_view>(rust::Str{"2020"}) == "2020");
#endif
r_take_slice_char(rust::Slice<const char>(SLICE_DATA, sizeof(SLICE_DATA)));
r_take_rust_string(rust::String("2020"));
r_take_empty_rust_string(rust::String{""});
r_take_empty_rust_string(rust::String{nullptr, 0});
#ifdef CXXBRIDGE_HAS_STRING_VIEW
r_take_rust_string(std::string_view{"2020"});
r_take_empty_rust_string(std::string_view{});
r_take_empty_rust_string(std::string_view{""});
ASSERT(static_cast<std::string_view>(rust::String{"2020"}) == "2020");
#endif
r_take_unique_ptr_string(
std::unique_ptr<std::string>(new std::string("2020")));
r_take_ref_vector(std::vector<uint8_t>{20, 2, 0});
Expand Down