std::mdspan (C++23)

In a nutshell

The multidimensional generalization of std::span: a multidimensional view over one contiguous block of memory. It does not own the data, its dimensions can be fixed at compile time or given at runtime, and indexing m[i, j] is zero-overhead.

Header file

#include <mdspan>

Core API Cheat Sheet

Operation	Signature	Description
Fixed extents	mdspan<T, extents<size_t, R, C>>	Rows and columns known at compile time
Dynamic extents	mdspan<T, dextents<size_t, Rank>>	Dimensions given at runtime
Indexing	m[i, j]	Multidimensional subscript (C++23 P2128) — note it is [], not ()
Row count	m.extent(0)	Size of dimension 0
Column count	m.extent(1)	Size of dimension 1
Total elements	m.size()	Product of all dimensions
Rank	m.rank()	How many dimensions
Underlying pointer	m.data_handle()	Get the raw pointer
Layout	layout_right (default) / layout_left / layout_stride	Row-major / column-major / custom strides

Minimal Example

// Standard: C++23
#include <mdspan>
#include <cstdio>

int main() {
    int data[2 * 3] = {1, 2, 3, 4, 5, 6};

    // Fixed extents: rows and columns known at compile time
    std::mdspan<int, std::extents<std::size_t, 2, 3>> m(data);
    std::printf("rows=%zu cols=%zu  m[0,0]=%d m[1,2]=%d\n",
                m.extent(0), m.extent(1), m[0, 0], m[1, 2]);

    // Dynamic extents: rows and columns given at runtime
    std::mdspan<int, std::dextents<std::size_t, 2>> dm(data, 2, 3);
    std::printf("dm[1,1]=%d size=%zu\n", dm[1, 1], dm.size());
}

Actual output (GCC 16.1.1, -std=c++23):

rows=2 cols=3  m[0,0]=1 m[1,2]=6
dm[1,1]=5 size=6

Embedded Suitability: Medium

• A zero-ownership view with no allocation: its size is just a pointer plus the extents (a few dozen bytes), which suits memory-constrained settings
• Multidimensional sensor data (images, matrices, ADC samples) can be passed around zero-copy, replacing hand-computed row*cols+col indexing
• The compile-time fixed-extent (extents) version can be optimized so thoroughly that everything lives in registers, with no runtime overhead
• Note that the multi-argument operator[] only exists as of C++23; the old operator() spelling found in older references no longer compiles on GCC 16

Compiler Support

GCC	Clang	MSVC
14	17	19.36

See Also

• cppreference: std::mdspan
• std::span — the one-dimensional view

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