169 lines
4.8 KiB
Markdown
169 lines
4.8 KiB
Markdown
# Modern C++ support
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Mapbox GL Native supports the C++14 standard, and encourages contributions to
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the source code using modern C++ idioms like return type deductions, generic
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lambdas, `std::optional` and alike. However, we do not support all the features
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from the final draft of the C++14 standard - we had to sacrifice support for
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these features in order to support GCC from version 4.9 onwards.
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The following C++14 features are **not supported** in Mapbox GL Native:
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## [C++14 variable templates](https://isocpp.org/wiki/faq/cpp14-language#variable-templates)
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Constructs like the example below are not supported:
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```C++
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template<typename T> constexpr T pi = T(3.14);
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```
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### Workarounds:
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- If the variable is an alias, use the call the alias points to: [example](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-711ce10b54a522c948efc9030ffab4fcL269)
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```C++
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// auto foo = pi<double>;
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auto foo = double(3.14);
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```
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- Replace variable templates with either functions or structs: [example 1](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-ffbe6cdfd30513aaa4749b4d959a5da6L58), [example 2](https://github.com/mapbox/mapbox-gl-native/commit/f1ac757bd28351fd57113a1e16f6c2e00ab193c1#diff-04af54dc8685cdc382ebe24466dc1d00L98)
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## [C++14 aggregates with non-static data member initializers](http://en.cppreference.com/w/cpp/language/aggregate_initialization)
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Constructs like the example below are not supported:
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```C++
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struct Foo {
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int x = { 0 };
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};
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// error: no matching function for call to 'Foo::Foo(<brace-enclosed initializer list>)'
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int main() {
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Foo foo { 0 };
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return 0;
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}
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```
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### Workarounds
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- Replace data member initializers with default parameter values in default constructors:
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```C++
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struct Foo {
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Foo(int x_ = 0) : x(x_) {}
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int x;
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};
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int main() {
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Foo foo { 0 }; // works with default constructor
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return 0;
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}
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```
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- Replace bracket initialization with regular round brackets or none:
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```C++
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struct Foo {
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Foo(int x_ = 0) : x(x_) {}
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int x;
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};
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int main() {
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Foo foo(); // works
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Foo bar; // also works
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return 0;
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}
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```
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## [Extended `constexpr` support](https://isocpp.org/wiki/faq/cpp14-language#extended-constexpr)
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GCC 4.9 strictly forbids `constexpr` usage in the following scenarios:
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- No local variable declarations (not `static` or `thread_local`, and no uninitialized variables)
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- Cannot mutate objects whose lifetime began with the constant expression evaluation
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- Disable usage of if, switch, for, while, do-while (not goto) inside constexpr expressions
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- Enforces that constexpr member functions are implicitly const
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```C++
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// sorry, unimplemented: use of the value of the object being constructed
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// in a constant expression
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struct Foo {
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int x, y;
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constexpr Foo(int i) : x(i), y(x) {}
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};
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// error: body of constexpr function 'constexpr int test1(int)' not a
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// return-statement
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constexpr int test1(int i) {
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int j = i;
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return j;
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}
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// error: body of constexpr function 'constexpr bool test2(int)' not a
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// return-statement
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constexpr bool test2(int i) {
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if (i > 0) {
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return true;
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} else {
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return false;
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}
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}
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```
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### Workarounds
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- Either remove `constexpr` specifier or replace it with `inline` in case of
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functions
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## [Polymorphic lambdas](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68278)
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GCC 5.2.0 crashes with polymorphic lambdas and this version of the compiler
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is currently used in Qt Automotive. Luckily polymorphic lambdas are rarely
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used/needed but we had one incident fixed by #9665.
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### Workarounds
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- Copy & Paste™ the code.
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## [Inheriting Constructors](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2540.htm)
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Compilers with only partial C++11 support causes compilation errors when using-declaration is used in derived class to inherit its base class's constructors `(e.g. using Base::Base;)`.
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```C++
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#include <vector>
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template <typename T>
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struct Varargs : std::vector<T>
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{
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using std::vector<T>::vector;
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};
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// error: conflicts with version inherited from 'std::__1::vector<int, std::__1::allocator<int> >'
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int main()
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{
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Varargs<int> v;
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return 0;
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}
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```
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### Workarounds
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- Replace using-declaration (e.g. using Base::Base;) in derived class with `universal forwarding constructor`.
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```C++
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#include <vector>
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template <typename T>
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struct Varargs : std::vector<T>
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{
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template <class... Args>
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Varargs(Args&&... args) : std::vector<T>(std::forward<Args>(args)...) {}
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};
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int main()
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{
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Varargs<int> v;
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return 0;
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}
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```
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Note: Using `universal forwarding constructor` may not be appropriate when derived class has additional members that are not in base class. Write constructors for the derived class explicitly instead of using `universal forwarding constructor` when the derived class has additional members.
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