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Diffstat (limited to 'gcc/doc/extend.texi')
| -rw-r--r-- | gcc/doc/extend.texi | 27 |
1 files changed, 24 insertions, 3 deletions
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index d9efab9..d3fc2d8 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -1759,7 +1759,8 @@ ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports -compound literals in C90 mode and in C++. +compound literals in C90 mode and in C++, though the semantics are +somewhat different in C++. Usually, the specified type is a structure. Assume that @code{struct foo} and @code{structure} are declared as shown: @@ -1785,8 +1786,9 @@ This is equivalent to writing the following: @} @end smallexample -You can also construct an array. If all the elements of the compound literal -are (made up of) simple constant expressions, suitable for use in +You can also construct an array, though this is dangerous in C++, as +explained below. If all the elements of the compound literal are +(made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here: @@ -1822,6 +1824,25 @@ static int y[] = @{1, 2, 3@}; static int z[] = @{1, 0, 0@}; @end smallexample +In C, a compound literal designates an unnamed object with static or +automatic storage duration. In C++, a compound literal designates a +temporary object, which only lives until the end of its +full-expression. As a result, well-defined C code that takes the +address of a subobject of a compound literal can be undefined in C++. +For instance, if the array compound literal example above appeared +inside a function, any subsequent use of @samp{foo} in C++ has +undefined behavior because the lifetime of the array ends after the +declaration of @samp{foo}. As a result, the C++ compiler now rejects +the conversion of a temporary array to a pointer. + +As an optimization, the C++ compiler sometimes gives array compound +literals longer lifetimes: when the array either appears outside a +function or has const-qualified type. If @samp{foo} and its +initializer had elements of @samp{char *const} type rather than +@samp{char *}, or if @samp{foo} were a global variable, the array +would have static storage duration. But it is probably safest just to +avoid the use of array compound literals in code compiled as C++. + @node Designated Inits @section Designated Initializers @cindex initializers with labeled elements |