static_any<S>

A container for generic (as general) data type — like boost.any. However:

It is ~10x faster than boost.any, mainly because there is no memory allocation
As it lies on the stack, it is cache-friendly, close to your other class attributes
There is a very small space overhead: a fixed overhead of 8 bytes

static_any<S> is also safe:

operations meet the strong exception guarantee
compile time check during the assignment, to ensure that its buffer is big enough to store the value
runtime check before any conversions, to ensure that the stored type is the one's requested by the user

Example

    static_any<32> a;
    static_assert(sizeof(a) == 32 + 8, "any has a fixed overhead of 8 bytes");

    a = 1234;
    ASSERT_EQ(1234, a.get<int>());

    a = std::string("foobar");
    ASSERT_EQ("foobar", a.get<std::string>());

    try {
      // This will throw as a std::string has been stored
      std::cout << any_cast<int>(a);
    }
    catch(bad_any_cast& ex) {
    }

    struct A : std::array<char, 32> {};
    a = A();

    struct B : A { char c; };  
    
    // Does not build: sizeof(B) is too big for static_any<16>
    a = B();

static_any_t<S>

A container similar to static_any<S>, but for trivially copyable types only. The differences:

No space overhead
Faster
Unsafe: there is no check when you try to access your data

Benchmarks

As the main advantage of static_any(_t<S>) is speed, here is a comparison of assign/get operations on a POD/non-POD types — an integer and a std::string — between boost.any, QVariant, static_any<S> and static_any_t<S>.

assign a double

Test             Time (ns)
--------------------------
qvariant                36
boost.any               57
std::any                 4
static_any<8>            4
static_any_t<8>          0

get a double

Test             Time (ns)
--------------------------
qvariant                 9
boost.any               12
std::any                 9
static_any<8>            9
static_any_t<8>          6

assign a small struct

Test             Time (ns)
--------------------------
qvariant               294
boost.any               47
std::any                54
static_any<8>           12
static_any_t<8>          0

get a small struct

Test             Time (ns)
--------------------------
qvariant                12
boost.any               48
std::any                 6 
static_any<8>            4
static_any_t<8>          0

assign a string

Test             Time (ns)
--------------------------
qvariant               400
boost.any              143
std::any               139
static_any<32>          92

get a string

Test             Time (ns)
--------------------------
qvariant                31
boost.any               64
std::any                30
static_any<32>           4

What I used for this benchmark:

Boost 1.54
Qt5
GCC 5.3.0
CPU i7-3537U

The code is available in benchmark.cpp. There is a dependency on geiger, a benchmarking library I developed. In order to build and run the benchmark, first install geiger:

git clone https://github.com/david-grs/geiger
cd geiger && mkdir build && cd build
cmake .. && make install

Then pass -DBUILD_BENCHMARK=1 to your cmake command.

david-grs/static_any

static_any<S>

Example

static_any_t<S>

Benchmarks