SV——类型转换$cast

0. 介绍

在SV中类型转换有很多，在这里先将类型转换分成两种，静态类型转换和动态转换。

静态转换就是用cast operator——单引号（‘）。

动态转换用$cast。

1. 静态转换 static cast（’）

语法如下：

constant_cast ::= // from A.8.4(来自1800)
casting_type ' ( constant_expression )

如果casting_type和表达式类型相同，那么会返回casting_type类型的值给到左值。如果类型不匹配，也会强行转换（string类型也会转换成int类型），但会出现问题：

casting_type是枚举类型，转换可能出界。

typedef enum {RED,BLUE,GREEN} color_e;
color_e color;
int c;
initial begin
    c=10;
    color=color_e'(c);
end

上面的转换会成功，color的值变成10，但这除了color_e这个枚举类型的界限了。

typedef enum {RED,BLUE,GREEN} COLOR_E;
COLOR_E color;
int c;
string str;
initial begin
  str="hh";
  c=int'(str);
  $display("c is %0d",c);
  c=1;
  color=COLOR_E'(c);
  $display("Color is %d / %s",color,color.name);
  c=3;
  color=COLOR_E'(c);
  $display("Color is %d / %s",color,color.name);
end
// 输出
c is 26728
Color is           1 / BLUE
Color is           3 /

从上面输出可以看出string类型转换成了int值；第二个color越界了。

casting_type是bit_stream type。
比特流类型先不整理了，没遇到过，在1800 6.24.3节，如果以后遇到再看吧。

2. 动态转换（$cast）

上面static cast不会检查表达式的类型，可能会发生string转换成int，枚举越界等问题。

动态转换$cast可以作为function，也可以作为task（这个问题地平线一面的时候被问过，$cast是函数还是任务），语法如下：

function int $cast( singular dest_var, singular source_exp );
or
task $cast( singular dest_var, singular source_exp );

Use of $cast as either a task or a function determines how invalid assignments are handled.

When called as a task, $cast attempts to assign the source expression to the destination variable. If the

assignment is invalid, a run-time error occurs, and the destination variable is left unchanged.

When called as a function, $cast attempts to assign the source expression to the destination variable and returns 1 if the cast is legal. If the cast fails, the function does not make the assignment and returns 0. When called as a function, no run-time error occurs, and the destination variable is left unchanged.

It is important to note that $cast performs a run-time check. No type checking is done by the compiler, except to check that the destination variable and source expression are singulars.

至于$cast是作为function还是task，主要看$cast使用的语境是否需要返回值。

$cast作为function

typedef enum {RED,BLUE,GREEN} COLOR_E;
COLOR_E color;
int c;
initial begin
  c=1;
  if(!$cast(color,c))
    $display("cast failed for c=%0d",c);
  $display("Color is %d / %s",color,color.name);
  c=3;
  if(!$cast(color,c))
    $display("cast failed for c=%0d",c);
  $display("Color is %d / %s",color,color.name);
end
// 输出
Color is           1 / BLUE
cast failed for c=3
Color is           1 / BLUE

第二次$cast失败之后，color的值不变。

$cast作为task

typedef enum {RED,BLUE,GREEN} COLOR_E;
COLOR_E color;
int c;
initial begin
  c=1;
  if(!$cast(color,c))
    $display("cast failed for c=%0d",c);
  $display("Color is %d / %s",color,color.name);
  c=3;
    $cast(color,c); // 作为任务，没有返回值的语境
  $display("Color is %d / %s",color,color.name);
end
// 会报错
Error-[STASKE_DCF] Dynamic cast failed
./svt.sv, 108
  Dynamic cast using '$cast' failed. The source expression is not yielding a
  valid value for the destination variable.

3. 其他类型转换

在IEEE 1800的20.5节中，还介绍了一些integer2string或者signed2unsigned的函数

$itor converts integer values to real values (for example, 123 becomes 123.0).

$realtobits converts values from a real type to a 64-bit vector representation of the real number.

$bitstoreal converts a bit pattern created by $realtobits to a value of the real type.

$shortrealtobits converts values from a shortreal type to the 32-bit vector representation of the real number.

$bitstoshortreal converts a bit pattern created by $shortrealtobits to a value of the shortreal type

$signed—returned value is signed

$unsigned—returned value is unsigned

module driver (net_r);
output [64:1] net_r;
real r;
wire [64:1] net_r = $realtobits(r);
endmodule
module receiver (net_r);
wire [64:1] net_r;
input [64:1] net_r;
real r;
initial assign r = $bitstoreal(net_r);
endmodule