Preserving Mathematica expressions in a textual form -

what proper way convert mathematica expressions losslessly string (a string kept in memory, not exported file)?

i looking textual representation that

1. will preserve information, including keeping special (and possibly atomic) objects, such sparsearray, graph, dispatch, compiledfunction, etc. intact. e.g. cycling sparsearray through representation should keep sparse (and not convert normal list).
2. is relatively fast cycle through (convert , forth).

is tostring[expr, fullform] sufficient this? tostring[expr, inputform]?

note 1: came while trying work around bugs in graph internal representation gets corrupted occasionally. i'm interested in answer general question above.

note 2: save surely this, writes files (probably possible solve using streams), , write definitions associated symbols.

if not going perform string manipulations on resulting string, may consider compress , uncompress alternative tostring. while don't know cases tostring[expr,inputform] - toexpression cycles break, can imagine exist. compress solution seems more robust, uncompress invoked on compress-ed string guaranteed reconstruct original expression. added advantage of compress is pretty memory-efficient - used few times save large amounts of numerical data in notebook, without saving them disk.