Why Can't You Use `std::string` in a C Union?

Why Doesn't the Compiler Allow std::string in Unions?

When working with C , it's often necessary to use data structures that can store different types of data. One such structure is a union, which assigns multiple variables to the same memory location. However, a common question arises regarding the limitations unions have on their members.

One significant restriction is the inability to include a class with a non-trivial copy constructor in a union. This includes classes like std::string. To understand why, consider the following scenarios:

union U {
  std::string x;
  std::vector<int> y;
};

U u; // <--

Traditionally, in a struct, initializing u.x or u.y would set them to default values. However, in a union, all members share the same address, making it impossible to assign values to both members without overwriting data. If neither member is initialized, using them would result in undefined behavior.

C 98 addressed this issue by prohibiting classes with non-trivial constructors from being members of unions. Specifically, it stated in §9.5/1:

"An object of a class with a non-trivial constructor (12.1), a non-trivial copy constructor (12.8), a non-trivial destructor (12.4), or a non-trivial copy assignment operator (13.5.3, 12.8) cannot be a member of a union, nor can an array of such objects."

This restriction ensures that data manipulation within unions can be handled effectively.

C 0x relaxed this rule to some extent (§9.5/2), allowing at most one non-static data member with a brace-or-equal initializer. However, writing error-free copy constructors and destructors for unions is still complex. For this reason, tagged unions or third-party libraries like boost::variant and boost::any provide alternative solutions for working with heterogeneous data.

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