On 10 mai, 06:57, terminator <farid.mehr...@[EMAIL PROTECTED]
> wrote:
> On May 9, 7:28 pm, Dan Smithers <dsmith...@[EMAIL PROTECTED]
> wrote:
> > What constitutes a constant-expression? I know that it is
> > something that can be determined at compile time.
> > I am trying to use template code and keep getting compiler
> > errors "error: cannot appear in a constant-expression"
> > template <int s>
> > class CFoo
> > {
> > private:
> > int m_val[s];
> > };
> > struct SParams
> > {
> > const int m_sz;
> > SParams(int sz) : m_sz(sz) {;}
> > };
> > int main(int argc, char *argv[])
> > {
> > CFoo<2> foo;
> > const int sz(2);
> > CFoo<sz> foo2;
> > int sz2(3);
> > CFoo<sz2> foo3;
> > const SParams params(4);
> > CFoo<params.m_sz> foo4;
> > }
> > When I compile this code I get the following output.
> > dwhs1@[EMAIL PROTECTED]
g++ -o template template.cpp
> > template.cpp:22: error: =91sz2=92 cannot appear in a
constant-expression=
> > template.cpp:22: error: template argument 1 is invalid
> > template.cpp:22: error: invalid type in declaration before =91;=92
token=
> > template.cpp: In function =91int main(int, char**)=92:
> > template.cpp:25: error: =91params=92 cannot appear in a
constant-express=
ion
> > template.cpp:25: error: `.' cannot appear in a constant-expression
> > template.cpp:25: error: template argument 1 is invalid
> > template.cpp:25: error: invalid type in declaration before =91;=92
token=
> > I understand that in line 22 I am using a local variable
> > that is not const, but on line 25 there is a const member of
> > a const structure.
> constant exp and const object are different matters.the former is
> usually refered to as literal or internally-linked value while the
> second is called a read-only object.
> A const object is that which is constructed at run-time and constant
> since construction until destruction.
> A constant exp is a value determined at compile-time.
I might add that in C++, a const object of integral type *can*
be used in a constant expression, but only if its initializers
are visible and constant expressions.
--
James Kanze (GABI Software) email:james.kanze@[EMAIL PROTECTED]
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