std::expected And Why It's Awesome
std::expected and some spectacular extensions are hopefully coming to C++23.
Existing Practice
I’m used to seeing code that looks like this, not just in C but in C++ codebases too:
int main() {
int ierr;
double * mat = (double*)malloc(sizeof(double[100]));
ierr = dowork(mat, 10, 10);
check_error(ierr);
ierr = domorework(mat, 10, 10);
check_error(ierr);
free(mat);
}
Integers represent error conditions, and these usually map to an error message like so:
const char* error_messages[] = {
"success",
"got some failure",
"another kind of failure",
};
enum error_types {
success,
some_failure,
some_other_failure,
};
void check_error(int ierr) {
if (ierr) {
printf("got error '%s'\n", error_messages[ierr]);
exit(ierr);
}
}
This way when you encounter an error condition in some function, you just return the corresponding error code like so:
int dowork(double* matrix, int M, int N) {
// do some work here
if (/*some error condition*/)
return some_failure;
// success state
return success;
}
And the error handler reports the failures:
Program returned: 1
got error 'got some failure'
On one hand, there’s a sense of security to writing code like this. You always check your error conditions, your code never throws exceptions, and the range of possible failures is very clear. I don’t mind this pattern in C, but in C++ we have some goodies that are far nicer to use in my opinion (especially in C++23).
Why std::expected is Awesome
I’ll be using mdspan from the Kokkos implementation and expected from Sy Brand’s implementation for this section.
In the last year, 3 papers have come through the ISO C++ mailing lists for std::expected, and the most recent paper from Jeff Garland proposes some of Sy Brand’s wonderful extensions to std::expected which allow you to chain together monadic operations on expected values:
and_thenor_elsetransform
I think these extensions are extremely elegant, and I think some folks that are more used to the C-style error handling could be won over.
Using std::expected means your errors have value semantics, which is something I like about the C-stlye error handling.
Chaining together these operations makes the programmer’s intent so much clearer.
Let’s look at another example using matrices, but this time using expected and mdspan.
expected Example
I’ll get some using statements out of the way:
namespace stdex = std::experimental;
using mat_t = stdex::mdspan<double,
stdex::extents<
stdex::dynamic_extent,
stdex::dynamic_extent
>
>;
using expect_mat = tl::expected<mat_t, std::string>;
I can’t help but marvel at how nice and readable this looks: The intent of the programmer is very clear in my opinion, even without seeing the rest of the code.
int main() {
auto raw = std::make_unique<double[]>(25);
auto mat = mat_t(raw.get(), 5, 5);
setup(mat) // zero initialize
.and_then(set_diag) // set the diagonal of the matrix
.and_then(print) // print out some values
.or_else(report_errors); // if unexpected, print the error and quit
}
/*
* Program returned: 0
* 1.0 0.0 0.0 0.0 0.0
* 0.0 1.0 0.0 0.0 0.0
* 0.0 0.0 1.0 0.0 0.0
* 0.0 0.0 0.0 1.0 0.0
* 0.0 0.0 0.0 0.0 1.0
*/
This leads to some nice and/or interesting patterns.
Say we want to check that the matrix passed to set_diag is square;
we could perform our check and return an error message if the check fails, much like we could throw an exception:
auto set_diag(mat_t mat) {
if (mat.extent(0) != mat.extent(1))
return make_unexpected("expected square matrix!");
for (int i=0; i < mat.extent(0); i++)
mat(i, i) = 1.0;
return expect_mat(mat);
}
I also like using an immediatly invoked lambda for this, but I’m not sure how readable/maintainable this is long-term:
auto set_diag(mat_t mat) {
return mat.extent(0) == mat.extent(1)
? [=] {
for (int i=0; i < mat.extent(0); i++)
mat(i, i) = 1.0;
return expect_mat(mat);
}()
: make_unexpected("expected square matrix!");
}
Either way, the error is handled as expected (no pun intended):
auto report_errors(std::string_view err) {
fmt::print("got error: '{}'\n", err);
std::exit(EXIT_FAILURE);
}
int main() {
auto raw = std::make_unique<double[]>(25);
// It's not square!
auto mat = mat_t(raw.get(), 25, 1);
setup(mat)
.and_then(set_diag)
.and_then(print)
.or_else(report_errors);
}
/*
* Program returned: 1
* got error: 'expected square matrix!'
*/
3 Ways to use the Monadic Functions on an Expected Value
1. Functor
We can use functors in the expected chain like so:
struct SetRow {
std::size_t row;
double value;
expect_mat operator()(mat_t mat) {
for (int i=0; i<mat.extent(1); i++)
mat(row, i) = value;
return mat;
}
};
int main() {
auto raw = std::make_unique<double[]>(25);
auto mat = mat_t(raw.get(), 5, 5);
setup(mat)
.and_then(set_diag)
.and_then(SetRow{1, 3.5})
.and_then(print)
.or_else(report_errors);
}
/*
* Program returned: 0
* 1.0 0.0 0.0 0.0 0.0
* 3.5 3.5 3.5 3.5 3.5
* 0.0 0.0 1.0 0.0 0.0
* 0.0 0.0 0.0 1.0 0.0
* 0.0 0.0 0.0 0.0 1.0
*/
2. Binding Args to a Function that Takes Multiple Arguments
Using std::bind on a function taking more arguments would also acomplish this:
auto set_row(mat_t mat, std::size_t row, double value) {
for (int i=0; i<mat.extent(1); i++)
mat(row, i) = value;
return expect_mat(mat);
}
int main() {
auto raw = std::make_unique<double[]>(25);
auto mat = mat_t(raw.get(), 5, 5);
setup(mat)
.and_then(set_diag)
.and_then(std::bind(set_row, /*mat=*/_1, /*row=*/1, /*value=*/3.5))
.and_then(print)
.or_else(report_errors);
}
/*
* Program returned: 0
* 1.0 0.0 0.0 0.0 0.0
* 3.5 3.5 3.5 3.5 3.5
* 0.0 0.0 1.0 0.0 0.0
* 0.0 0.0 0.0 1.0 0.0
* 0.0 0.0 0.0 0.0 1.0
*/
3. Lambdas
And of course, lambdas:
int main() {
auto raw = std::make_unique<double[]>(25);
auto mat = mat_t(raw.get(), 5, 5);
setup(mat)
.and_then(set_diag)
.and_then([] (auto mat) {
for (int i=0; i < mat.extent(1); i++)
mat(3, i) = 2.0;
return expect_mat(mat);
})
.and_then(print)
.or_else(report_errors);
}
/*
* Program returned: 0
* 1.0 0.0 0.0 0.0 0.0
* 0.0 1.0 0.0 0.0 0.0
* 0.0 0.0 1.0 0.0 0.0
* 2.0 2.0 2.0 2.0 2.0
* 0.0 0.0 0.0 0.0 1.0
*/
Conclusion
Hopefully you’ve been won over by the elegance of expected and mdspan.
Godbolt links can be found for these examples in the links below: