#How to conditionally reference variables using structured bindings?

5 messages · Page 1 of 1 (latest)

warm sparrow
#

In the following code in function deliver_image I try to conditionally reference either the passed parameters or some new stack variables. It doesn't compile like this, how can I work around this? The idea is that the function parameters can't be altered, so it needs to make a copy, but only if a certain condition (int c == 4) is true.

Godbolt link: https://godbolt.org/z/P7GssedEx

#include <utility>
#include <iostream>
#include <string>
#include <tuple>

using MATRIX = std::string;
using ColorMap = std::string;

volatile int c = 4;

auto shift_vals(const MATRIX& map, const ColorMap& cm) -> std::pair<MATRIX, ColorMap>{
MATRIX new_map;
ColorMap new_cm;
return {new_map, new_cm};
}

auto deliver_image(MATRIX& mapgrid, ColorMap colormap) {

MATRIX new_map;
ColorMap new_cm;
auto& [map, cm] = [&](){
    if (c == 4) {
        std::tie(new_map, new_cm) = shift_vals(mapgrid, colormap);
        return std::pair<MATRIX&, ColorMap&>(new_map, new_cm);
    } else {
        return std::pair<MATRIX&, ColorMap&>(mapgrid, colormap);
    }
}();

}

int main() {

MATRIX mapgrid;
ColorMap cm;
deliver_image(mapgrid, cm);

}

Compiler Explorer - C++ (x86-64 gcc 13.2)

using MATRIX = std::string;
using ColorMap = std::string;

volatile int c = 4;

auto shift_vals(const MATRIX& map, const ColorMap& cm) -> std::pair<MATRIX, ColorMap>{
MATRIX new_map;
ColorMap new_cm;
return {new_map, new_cm};
}

auto deliver_image(MATRIX& mapgrid, ColorMap colormap) {

MATRIX new_map;
ColorMap new_cm;
aut...
royal chasmBOT
#

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winged pewter
#

Remove the & from auto &[map, cm].

#

The & refers to the pair not to the MATRIX and ColorMap.

warm sparrow