Variant to replace a union but is “undeclared identifier”

[stephanos]

Greetings from sunny London. I know some C, have written in Lazarus Pascal, enjoy coding for pleasure, and decided to dabble in C++. Lots of terminology still escapes me.

I was experimenting with structures and unions and quickly realised that unions do not handle strings well and according to web searches, I should consider using “#include <variant>” and “std::variant<int, std::string> var; var = "hello";”
But the latter line produces a series of errors which seem to cluster around “C2039 ‘variant’: is not a member of ‘std’”
and
“C2065 ‘variant’: undeclared identifier”

un-commenting using namespace std; does not improve the situation.

Any help appreciated

Stephanos

---

// StructuresV2-2.cpp
// OS - Windows 10 Enterprise
// Compiler - C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.26.28801\bin\Hostx64\x64>cl.exe
// Microsoft(R) C / C++ Optimizing Compiler Version 19.26.28806 for x64
// Copyright(C) Microsoft Corporation.All rights reserved.
// usage: cl[option...] filename...[/ link linkoption...]
// C : \Program Files(x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.26.28801\bin\Hostx64\x64 >
// Application - MS Visual Studio Community 201916.6.3
// Experiement to see if a union can be a field in a structure
// It can.  But unions limited ability to store/use string.
// Use: 
//   #include <variant>
//   std::variant 
// to replace union
using namespace std;
#include <iostream>
#include <variant>
#include <string>  // needed to use std::to_string()
struct coffeeBean
{
    std::string name; std::string country; int strength; union internationalgrade;
} newBean; //
int main()
{
    newBean.name = "Flora";
    newBean.country = "Mexico";
    newBean.strength = 9;
    std::cout << "Bean: " + newBean.name + " From: " + newBean.country;
    std::cout << " Strength: " + std::to_string(newBean.strength) << std::endl;
    std::variant<int, std::string> var; var = "hello";

    return 0;
}

[Salem]

std::variant - cppreference.com
You need to tell your compiler to compile in C++17 mode.

[stephanos]

Dear Salem

Thank you

But how? This application, MS Visual Studio Community 201916.6.3 is harder to use than Code::Blocks. I have looked around and cannot see how to do that.

In Code::Block the compiler is GNU GCC Compiler. There are 3 that start Microsoft Visual C++, 1) Toolkit 2003, 2) 2005-2008, 3) 2010. If I chose either of these I will not know how to change the Toolchain Executable tab.

Thanks

Stephanos

[Salem]

c++ - How to enable C++17 compiling in Visual Studio? - Stack Overflow

Or
g++ -std=c++17 prog.cpp

[stephanos]

Dear Salem

Thanks, I made the change and it builds. Next I added a cout:

std::cout << "Content of var is: " + std::to_string(var);

but this led to an error:
“C2665 'std::to_string': none of the 9 overloads could convert all the argument types”
So I decided to see if an integer would print out. I changed the value of ‘var’

std::variant<int, std::string> var; var = 99;

which did build, and added a cout

std::cout << "Content of var is: " + std::to_string(var);

I got the same error message:
“C2665 'std::to_string': none of the 9 overloads could convert all the argument types”

Am I wasting my time? Eventually I want to use a variant inside a structure. If this is silly because I do not know something I am happy to abandon the plan.

Any further help appreciated

Stephanos

[Salem]

Maybe say which variant you mean.

std::cout << "Content of var is: " + std::to_string(std::get<int>(var));

[stephanos]

Dear Salem

Thanks, this is now solved thanks to your last suggestion.
Here is the code to declare a variant, assign integer data to it and then print out

std::variant<int, std::string> var; var = 99; 
std::cout << "Content of var is: " + std::to_string(std::get<int>(var));

Here is the code to declare a variant, assign a string data to it and then print out

std::variant<int, std::string> var; var = "Hello";
std::cout << "Content of var is: " + std::get<std::string>(var);

I have learned much, especially about ‘get’ and putting things in <>.

May all your code run smoothly

Stephanos