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# Modern C++ support
Mapbox GL Native supports the C++14 standard, and encourages contributions to
the source code using modern C++ idioms like return type deductions, generic
lambdas, `std::optional` and alike. However, we do not support all the features
from the final draft of the C++14 standard - we had to sacrifice support for
these features in order to support GCC from version 4.9 onwards.
The following C++14 features are **not supported** in Mapbox GL Native:
## [C++14 variable templates](https://isocpp.org/wiki/faq/cpp14-language#variable-templates)
Constructs like the example below are not supported:
```C++
template<typename T> constexpr T pi = T(3.14);
```
### Workarounds:
- 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)
```C++
// auto foo = pi<double>;
auto foo = double(3.14);
```
- 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)
## [C++14 aggregates with non-static data member initializers](http://en.cppreference.com/w/cpp/language/aggregate_initialization)
Constructs like the example below are not supported:
```C++
struct Foo {
int x = { 0 };
};
// error: no matching function for call to 'Foo::Foo(<brace-enclosed initializer list>)'
int main() {
Foo foo { 0 };
return 0;
}
```
### Workarounds
- Replace data member initializers with default parameter values in default constructors:
```C++
struct Foo {
Foo(int x_ = 0) : x(x_) {}
int x;
};
int main() {
Foo foo { 0 }; // works with default constructor
return 0;
}
```
- Replace bracket initialization with regular round brackets or none:
```C++
struct Foo {
Foo(int x_ = 0) : x(x_) {}
int x;
};
int main() {
Foo foo(); // works
Foo bar; // also works
return 0;
}
```
## [Extended `constexpr` support](https://isocpp.org/wiki/faq/cpp14-language#extended-constexpr)
GCC 4.9 strictly forbids `constexpr` usage in the following scenarios:
- No local variable declarations (not `static` or `thread_local`, and no uninitialized variables)
- Cannot mutate objects whose lifetime began with the constant expression evaluation
- Disable usage of if, switch, for, while, do-while (not goto) inside constexpr expressions
- Enforces that constexpr member functions are implicitly const
```C++
// sorry, unimplemented: use of the value of the object being constructed
// in a constant expression
struct Foo {
int x, y;
constexpr Foo(int i) : x(i), y(x) {}
};
// error: body of constexpr function 'constexpr int test1(int)' not a
// return-statement
constexpr int test1(int i) {
int j = i;
return j;
}
// error: body of constexpr function 'constexpr bool test2(int)' not a
// return-statement
constexpr bool test2(int i) {
if (i > 0) {
return true;
} else {
return false;
}
}
```
### Workarounds
- Either remove `constexpr` specifier or replace it with `inline` in case of
functions
## [Polymorphic lambdas](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68278)
GCC 5.2.0 crashes with polymorphic lambdas and this version of the compiler
is currently used in Qt Automotive. Luckily polymorphic lambdas are rarely
used/needed but we had one incident fixed by #9665.
### Workarounds
- Copy & Paste™ the code.
|
