emplace_back is pushing the structure into my array (C++)

May 9, 2024

I’m practicing pushing and popping. For this I’m using the emplace_back function to push a struct into my std::vector.

Here’s my code:

#include <string>
#include <vector>
#include <iostream>
#include <string_view>

struct Person
{
    std::string name{};
    int age{};
};

int main()
{
    std::vector<Person> nums{{"Daniel", 34},
                             {"Jose",   39},
                             {"Martin", 22}
                              };

    nums.emplace_back("Malena", 30);


    for (auto const& a : nums)
        std::cout << a.name << " " << a.age << '\n';

    return 0;
}

Error:

In file included from /Users/xxx/CLionProjects/test/main.cpp:1:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/string:504:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/string_view:175:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/__string:57:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:643:
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1881:31: error: no matching constructor for initialization of 'Person'
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/memory:1773:18: note: in instantiation of function template specialization 'std::__1::allocator<Person>::construct<Person, char const (&)[7], int>' requested here
            {__a.construct(__p, _VSTD::forward<_Args>(__args)...);}

Any help is appreciated.

>Solution :

This requires C++20. Some options in C++17 or older are:

Define a constructor for Person:

struct Person
{
    Person(const std::string& name, int age)
       : name(name)
       , age(age)
    {}

    std::string name;
    int age = 0;
};

Pass a constructed object explicitly (C++14):
```
nums.emplace_back(Person{"Malena", 30});
```
Use push_back instead with a braced initializer list (C++14):
```
nums.push_back({"Malena", 30});
```