algoProg algoProg - 1 year ago 93
C++ Question

Vector<tuple>, make_tuple<int, ignore, float>

I create a vector of tuples:

std::vector<std::tuple<int*, bool, int*>> *DataStucture;

Next I want to iterate over a data set to get only the first element of each tuple.

Is this legal:

DataStructure -> push_back(std::make_tuple(some_pointer_to_some_int_value, std::ignore, std::ignore));

In next round of data set scanning, I compare the value of some_int_value and upon matching I set following two elements of DataStructure:

DataStructure -> push_back(std::make_tuple(std::ignore, some_bool_value, some_pointer_to_some_int_value2);

I am not sure what exactly
is for. I tried checking cpp reference website but I did not get it.

Answer Source

Just to elaborate on my previous comment, you cannot use std::ignore here, because it can be used only as lvalue, but really you don't need to either. Just use nullptr or any "default" value instead:

DataStructure->push_back(std::make_tuple(some_pointer_to_some_int_value, false, nullptr));

std::ignore should be used if you want to unpack your tuple into different values like this:

int* p; 
bool b; 
for (auto& tuple : *DataStructure) {
    std::tie(p,b,std::ignore) = tuple;
    // p now have value of first element of tuple
    // b now have value of second element of tuple

On an unrelated note, your second operation does not set values of existing tuple, it adds new one. Also, why use pointer to a vector instead of vector itself or at least std::unique_ptr? It is generaly considered better to omit unnecessary "naked" pointers.

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