All instances v of type variant<T1,T2,...,TN> guarantee that v has constructed content of one of the types Ti, even if an operation on v has previously failed.

This implies that variant may be viewed precisely as a union of exactly its bounded types. This "never-empty" property insulates the user from the possibility of undefined variant content and the significant additional complexity-of-use attendant with such a possibility.

While the never-empty guarantee might at first seem "obvious," it is in fact not even straightforward how to implement it in general (i.e., without unreasonably restrictive additional requirements on bounded types).

The central difficulty emerges in the details of variant assignment. Given two instances v1 and v2 of some concrete variant type, there are two distinct, fundamental cases we must consider for the assignment v1 = v2.

First consider the case that v1 and v2 each contains a value of the same type. Call this type T. In this situation, assignment is perfectly straightforward: use T::operator=.

However, we must also consider the case that v1 and v2 contain values of distinct types. Call these types T and U. At this point, since variant manages its content on the stack, the left-hand side of the assignment (i.e., v1) must destroy its content so as to permit in-place copy-construction of the content of the right-hand side (i.e., v2). In the end, whereas v1 began with content of type T, it ends with content of type U, namely a copy of the content of v2.

The crux of the problem, then, is this: in the event that copy-construction of the content of v2 fails, how can v1 maintain its "never-empty" guarantee? By the time copy-construction from v2 is attempted, v1 has already destroyed its content!

Upon learning of this dilemma, clever individuals may propose the following scheme hoping to solve the problem:

While complicated, it appears such a scheme could provide the desired safety in a relatively efficient manner. In fact, several early iterations of the library implemented this very approach.

Unfortunately, as Dave Abraham's first noted, the scheme results in undefined behavior:

Additionally, as close examination quickly reveals, the scheme has the potential to create irreconcilable race-conditions in concurrent environments.

Ultimately, even if the above scheme could be made to work on certain platforms with particular compilers, it is still necessary to find a portable solution.

Upon learning of the infeasibility of the above scheme, Anthony Williams proposed in [Wil02] a scheme that served as the basis for a portable solution in some pre-release implementations of variant.

The essential idea to this scheme, which shall be referred to as the "double storage" scheme, is to provide enough space within a variant to hold two separate values of any of the bounded types.

With the secondary storage, a copy the right-hand side can be attempted without first destroying the content of the left-hand side; accordingly, the content of the left-hand side remains available in the event of an exception.

Thus, with this scheme, the variant implementation needs only to keep track of which storage contains the content -- and dispatch any visitation requests, queries, etc. accordingly.

The most obvious flaw to this approach is the space overhead incurred. Though some optimizations could be applied in special cases to eliminate the need for double storage -- for certain bounded types or in some cases entirely (see the section called “Enabling Optimizations” for more details) -- many users on the Boost mailing list strongly objected to the use of double storage. In particular, it was noted that the overhead of double storage would be at play at all times -- even if assignment to variant never occurred. For this reason and others, a new approach was developed.

Despite the many objections to the double storage solution, it was realized that no replacement would be without drawbacks. Thus, a compromise was desired.

To this end, Dave Abrahams suggested to include the following in the behavior specification for variant assignment: "variant assignment from one type to another may incur dynamic allocation." That is, while variant would continue to store its content in situ after construction and after assignment involving identical contained types, variant would store its content on the heap after assignment involving distinct contained types.

The algorithm for assignment would proceed as follows:

Since all operations on pointers are nothrow, this scheme would allow variant to meet its never-empty guarantee.

The most obvious concern with this approach is that while it certainly eliminates the space overhead of double storage, it introduces the overhead of dynamic-allocation to variant assignment -- not just in terms of the initial allocation but also as a result of the continued storage of the content on the heap. While the former problem is unavoidable, the latter problem may be avoided with the following "temporary heap backup" technique:

With this technique: 1) only a single storage is used; 2) allocation is on the heap in the long-term only if the assignment fails; and 3) after any successful assignment, storage within the variant is guaranteed. For the purposes of the initial release of the library, these characteristics were deemed a satisfactory compromise solution.

There remain notable shortcomings, however. In particular, there may be some users for which heap allocation must be avoided at all costs; for other users, any allocation may need to occur via a user-supplied allocator. These issues will be addressed in the future (see the section called “Future Direction: Policy-based Implementation”). For now, though, the library treats storage of its content as an implementation detail. Nonetheless, as described in the next section, there are certain things the user can do to ensure the greatest efficiency for variant instances (see the section called “Enabling Optimizations” for details).

As described in the section called “The Implementation Problem”, the central difficulty in implementing the never-empty guarantee is the possibility of failed copy-construction during variant assignment. Yet types with nothrow copy constructors clearly never face this possibility. Similarly, if one of the bounded types of the variant is nothrow default-constructible, then such a type could be used as a safe "fallback" type in the event of failed copy construction.

Accordingly, variant is designed to enable the following optimizations once the following criteria on its bounded types are met:

Caveat: On most platforms, the Type Traits templates has_nothrow_copy and has_nothrow_constructor by default return false for all class and struct types. It is necessary therefore to provide specializations of these templates as appropriate for user-defined types, as demonstrated in the following:

// ...in your code (at file scope)...

namespace boost {

  template <>
  struct has_nothrow_copy< myUDT >
    : mpl::true_
  {
  };

}

Implementation Note: So as to make the behavior of variant more predictable in the aftermath of an exception, the current implementation prefers to default-construct boost::blank if specified as a bounded type instead of other nothrow default-constructible bounded types. (If this is deemed to be a useful feature, it will become part of the specification for variant; otherwise, it may be obsoleted. Please provide feedback to the Boost mailing list.)

As the previous sections have demonstrated, much effort has been expended in an attempt to provide a balance between performance, data size, and heap usage. Further, significant optimizations may be enabled in variant on the basis of certain traits of its bounded types.

However, there will be some users for whom the chosen compromise is unsatisfactory (e.g.: heap allocation must be avoided at all costs; if heap allocation is used, custom allocators must be used; etc.). For this reason, a future version of the library will support a policy-based implementation of variant. While this will not eliminate the problems described in the previous sections, it will allow the decisions regarding tradeoffs to be decided by the user rather than the library designers.