https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122259
Bug ID: 122259
Summary: Enhance assignment to a const member compiler error
message
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jg at jguk dot org
Target Milestone: ---
Enhancement suggestion.
This code gives 59 lines of C++ template output, and amonust that I couldn't
see an. Could it have a one line error?
file.cpp:7 error: assignment of const (read-only) typedef struct d_m_t 'test.a'
member 'a'
It's not real code, just something to show the issue. I based the suggested one
line error on the equivalent C const read-only warning.
The first line gives some clue:
<source>:12:14: error: no match for 'operator=' (operand types are 'const
std::string' {aka 'const std::__cxx11::basic_string<char>'} and 'std::string'
{aka 'std::__cxx11::basic_string<char>'})
12 | test.a = oops;
Clang (trunk) gives only 21 lines output, and the first line is brief, although
still not clear.
<clang_source>:12:12: error: no viable overloaded '='
12 | test.a = oops;
#include <string>
typedef struct d_m
{
std::string a;
} d_m_t;
int square(int num)
{
const d_m_t test;
std::string oops;
test.a = oops;
//const char * const p = nullptr;
//p = "hello";
return 0;
}
https://godbolt.org/z/jbo7Ko47G
Just to share what else I compared.
If I change to a C style struct, the error is brief, says const instead of
read-only
https://godbolt.org/z/77bj9GMsc
typedef struct d_m
{
const char * const buf;
} d_m_t;
int square(int num)
{
const d_m_t test;
test.buf = nullptr;
return 0;
}
<source>:17:14: error: cannot assign to non-static data member 'buf' with
const-qualified type 'const char *const'
17 | test.buf = nullptr;
| ~~~~~~~~ ^
<source>:6:24: note: non-static data member 'buf' declared const here
6 | const char * const buf;
| ~~~~~~~~~~~~~~~~~~~^~~
Lastly I tried as C.
Changing to use GCC to compile as C the error is clear.
<source>: In function 'square':
<source>:13:7: error: assignment of read-only variable 'p'
13 | p = "hello";
| ^
<source>:14:14: error: assignment of member 'buf' in read-only object
14 | test.buf = NULL;
| ^
<source>:12:24: warning: variable 'p' set but not used
[-Wunused-but-set-variable=]
12 | const char * const p = NULL;
| ^
<source>:10:17: warning: variable 'test' set but not used
[-Wunused-but-set-variable=]
10 | const d_m_t test;
| ^~~~
Compiler returned: 1
#include <stddef.h>
typedef struct d_m
{
const char * const buf;
} d_m_t;
int square(void)
{
const d_m_t test;
const char * const p = NULL;
p = "hello";
test.buf = NULL;
return 0;
}
https://godbolt.org/z/oe6sroG3a
